Method and apparatus for encoding images and method and apparatus for decoding

ABSTRACT

Provided are a method and an apparatus for encoding and decoding motion information of a current prediction unit that is motion-predicted. The image encoding method includes: obtaining a first reference picture list, a second reference picture list, and a combination reference picture list which is a combination of reference pictures included in the first reference picture list and reference pictures included in the second reference picture list; and encoding a reference syntax indicating a motion prediction mode and a reference picture used in encoding a current prediction unit based on a number of possible cases of a unidirectional motion prediction mode and a number of possible cases of a bidirectional motion prediction mode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage entry under 35 U.S.C. 371(c) ofInternational Patent Application No. PCT/KR2012/001793, filed Mar. 12,2012, and claims priority from U.S. Provisional Application 61/451,789,filed on Mar. 11, 2011, the disclosures of which are incorporated hereinby reference in their entireties.

BACKGROUND

1. Field

Apparatuses and methods consistent with exemplary embodiments relate tomethods and apparatuses for encoding and decoding a still image and avideo, and more particularly, to methods and apparatuses for encodingand decoding motion information of a current prediction unit.

2. Description of the Related Art

According to a codec such as MPEG-4 H.264/MPEG-4 advanced video coding(AVC), prediction mode information indicating a direction of motion of acurrent block during motion prediction and reference picture informationused in motion prediction of the current block are encoded by using aseparate syntax.

SUMMARY

One or more exemplary embodiments provide methods and apparatuses forencoding and decoding motion prediction mode information of a currentprediction unit and reference picture information used in motionprediction, by using a single syntax.

According to an aspect of an exemplary embodiment, there is provided animage encoding method comprising: obtaining a first reference picturelist, a second reference picture list, and a combination referencepicture list which is a combination of reference pictures included inthe first reference picture list and reference pictures included in thesecond reference picture list; encoding a current prediction unit byusing one of a unidirectional prediction mode in which unidirectionalprediction is performed with respect to the current prediction unit byusing a reference picture included in the combination reference picturelist and a bidirectional prediction mode in which bidirectional motionprediction is performed with respect to the current prediction unit byusing a combination of reference pictures included in the firstreference picture list and the second reference picture list; andencoding a reference syntax indicating a motion prediction mode and areference picture used in encoding the current prediction unit based ona number of possible cases of the unidirectional motion prediction modeand a number of possible cases of the bidirectional motion predictionmode.

According to an aspect of another exemplary embodiment, there isprovided an image encoding apparatus, comprising: a motion estimator forpredicting a current prediction unit by using one of a unidirectionalprediction mode where unidirectional motion prediction with respect to acurrent prediction unit is performed by using a reference pictureincluded in a combination reference picture list in which referencepictures of a first reference picture list and reference pictures of asecond reference picture list are combined, and a bidirectional motionprediction mode where bidirectional motion prediction with respect tothe current prediction unit is performed by using the first referencepicture list and the second reference picture list; and an entropyencoder for encoding a reference syntax indicating a motion predictionmode and a reference picture used in encoding of the current predictionunit based on a number of possible cases of the unidirectional motionprediction mode and a number of possible cases of the bidirectionalmotion prediction mode.

According to an aspect of still another exemplary embodiment, there isprovided an image decoding method comprising: obtaining a firstreference picture list, a second reference picture list, and acombination reference picture list which is a combination of referencepictures included in the first reference picture list and referencepictures included in the second reference picture list; determining avalue of a reference syntax according to a motion prediction mode andreference pictures used in encoding a current prediction unit based on anumber of possible cases of a unidirectional motion prediction modewhere reference pictures included in the combination reference list areused and a number of possible cases of a bidirectional motion predictionmode where a combination of reference pictures included in the firstreference picture list and the second reference picture list is used;obtaining a reference syntax of the current prediction unit from abitstream; determining a motion prediction mode and a reference pictureof the current prediction unit by using a value of the obtainedreference syntax; and performing motion compensation with respect to thecurrent prediction unit by using the determined motion prediction modeand the determined reference picture.

According to an aspect of still another exemplary embodiment, there isprovided an image decoder comprising: an entropy decoder for obtaining afirst reference picture list, a second reference picture list, and acombination reference picture list which is a combination of referencepictures included in the first reference picture list and referencepictures included in the second reference picture list, determining avalue of a reference syntax according to a motion prediction mode andreference pictures used in encoding of a current prediction unit basedon a number of possible cases of a unidirectional motion prediction modewhere reference pictures included in the combination reference list areused and a number of possible cases of a bidirectional motion predictionmode where the first reference picture list and the second referencepicture list are used, and determining a motion prediction mode and areference picture of the current prediction unit by using a referencesyntax of the current prediction unit obtained from a bitstream; and amotion compensation unit for performing motion compensation with respectto the current prediction unit by using the determined motion predictionmode and the determined reference picture.

BRIEF DESCRIPTION OF DRAWINGS

The above and/or other aspects will become more apparent by describingcertain exemplary embodiments with reference to the accompanieddrawings, in which:

FIG. 1 is a block diagram of a video encoding apparatus according to anexemplary embodiment;

FIG. 2 is a block diagram of a video decoding apparatus according to anexemplary embodiment;

FIG. 3 is a diagram for describing a concept of coding units accordingto an exemplary embodiment;

FIG. 4 is a block diagram of a video encoder based on coding unitshaving a hierarchical structure according to an exemplary embodiment;

FIG. 5 is a block diagram of a video decoder based on coding unitshaving a hierarchical structure according to an exemplary embodiment;

FIG. 6 is a diagram illustrating deeper coding units according to depthsand partitions according to an exemplary embodiment;

FIG. 7 is a diagram for describing a relationship between a coding unitand transformation units according to an exemplary embodiment;

FIG. 8 is a diagram for describing encoding information of coding unitscorresponding to a coded depth according to an exemplary embodiment;

FIG. 9 is a diagram of deeper coding units according to depths accordingto an exemplary embodiment;

FIGS. 10 through 12 are diagrams for describing a relationship betweencoding units, prediction units, and frequency transformation unitsaccording to an exemplary embodiment;

FIG. 13 is a diagram for describing a relationship between a codingunit, a prediction unit, and a transformation unit according to encodingmode information of Table 1;

FIG. 14 is a diagram illustrating an example of a reference picturereferred to by a prediction unit in a B picture according to anexemplary embodiment;

FIGS. 15A through 15C illustrate an example of a reference picture indexallocated to a reference picture according to an exemplary embodiment;

FIGS. 16A and 16B illustrate a combination reference picture list usedin unidirectional prediction according to exemplary embodiments;

FIG. 17 illustrates a table showing reference syntax values indicatingunidirectional and bidirectional motion prediction modes and referencepictures allocated based on a number of cases of unidirectional motionprediction modes and a number of cases of bidirectional motionprediction modes according to an exemplary embodiment;

FIG. 18 illustrates an example of a process of binarizing referencesyntax information according to an exemplary embodiment;

FIG. 19 is a flowchart illustrating an image encoding method accordingto an exemplary embodiment; and

FIG. 20 is a flowchart illustrating an image decoding method accordingto an exemplary embodiment.

DETAILED DESCRIPTION

Hereinafter, exemplary embodiments will be described in detail withreference to the accompanying drawings.

FIG. 1 is a block diagram of a video encoding apparatus 100 according toan exemplary embodiment.

The video encoding apparatus 100 includes a maximum coding unit splitter110, a coding unit determiner 120, and an output unit 130.

The maximum coding unit splitter 110 may split a current picture of animage based on a maximum coding unit, which is a coding unit of amaximum size. When the current picture is larger than the maximum codingunit, image data of the current picture may be split into at least onemaximum coding unit. The maximum coding unit according to an exemplaryembodiment may be a data unit having a size of 32×32, 64×64, 128×128,256×256, etc., wherein a shape of the data unit is a square which has awidth and a length in squares of 2 and is greater than 8. Image data maybe output to the coding unit determiner 120 in units of at least onemaximum coding unit.

A coding unit according to an exemplary embodiment may be characterizedby a maximum size and a depth. The depth denotes the number of times thecoding unit is spatially split from the maximum coding unit, and as thedepth deepens, deeper coding units may be split from the maximum codingunit to a minimum coding unit. A depth of the maximum coding unit is anuppermost depth and a depth of the minimum coding unit is a lowermostdepth. Since a size of a coding unit corresponding to each depthdecreases as the depth of the maximum coding unit deepens, a coding unitcorresponding to an upper depth may include a plurality of coding unitscorresponding to lower depths.

As described above, image data of the current picture may be split intothe maximum coding units according to a maximum size of the coding unit,and each of the maximum coding units may include deeper coding unitsthat are split according to depths. Since the maximum coding unitaccording to an exemplary embodiment is split according to depths, theimage data of a spatial domain included in the maximum coding unit maybe hierarchically classified according to depths.

A maximum depth and a maximum size of a coding unit, which limit thetotal number of times a height and a width of the maximum coding unitare hierarchically split, may be predetermined.

The coding unit determiner 120 encodes at least one split regionobtained by splitting a region of the maximum coding unit according todepths and determines a depth to output finally encoded image dataaccording to the at least one split region. In other words, the codingunit determiner 120 determines a coded depth by encoding the image datain the deeper coding units according to respective depths, according tothe maximum coding unit of the current picture, and selecting a depthhaving a least encoding error as the coded depth. The determined codeddepth and the encoded image data according to maximum coding unit areoutput to the output unit 130.

The image data in the maximum coding unit is encoded based on the deepercoding units corresponding to at least one depth equal to or smallerthan the maximum depth, and results of encoding the image data arecompared based on each of the deeper coding units. A depth having theleast encoding error may be selected after comparing encoding errors ofthe deeper coding units. At least one coded depth may be selected foreach maximum coding unit.

The size of the maximum coding unit is split as a coding unit ishierarchically split according to depths and as the number of codingunits increases. Also, when coding units of one maximum coding unitcorrespond to a same depth, it is individually determined whether tosplit each of the coding units corresponding to the same depth to alower depth by separately measuring an encoding error of the image dataof each coding unit. Accordingly, when image data is included in onemaximum coding unit, the image data may be split into regions accordingto the depths, and the encoding errors may differ according to regionsin the one maximum coding unit, and thus the coded depths may differaccording to regions in the image data. Thus, one or more coded depthsmay be determined in one maximum coding unit, and the image data of themaximum coding unit may be divided according to coding units of at leastone coded depth.

Accordingly, the coding unit determiner 120 may determine coding unitshaving a tree structure included in a current maximum coding unit. Thecoding units having a tree structure according to an exemplaryembodiment include coding units corresponding to a depth determined tobe the coded depth, from among all deeper coding units included in themaximum coding unit. A coding unit having a coded depth may behierarchically determined according to depths in the same region of themaximum coding unit and may be independently determined in differentregions. Similarly, a coded depth in a current region may beindependently determined from a coded depth in another region.

A maximum depth according to an exemplary embodiment is an index relatedto the number of times splitting is performed from a maximum coding unitto a minimum coding unit. A first maximum depth according to anexemplary embodiment may denote the total number of times splitting isperformed from the maximum coding unit to the minimum coding unit. Asecond maximum depth according to an exemplary embodiment may denote thetotal number of depth levels from the maximum coding unit to the minimumcoding unit. For example, when a depth of the maximum coding unit is 0,a depth of a coding unit, in which the maximum coding unit is splitonce, may be set to 1, and a depth of a coding unit, in which themaximum coding unit is split twice, may be set to 2. Here, when theminimum coding unit is a coding unit obtained when the maximum codingunit is split four times, five depth levels of depths 0, 1, 2, 3, and 4exist, and thus the first maximum depth may be set to 4, and the secondmaximum depth may be set to 5.

Prediction encoding and transformation may be performed according to themaximum coding unit. The prediction encoding and the transformation arealso performed based on the deeper coding units according to a depthequal to or less than the maximum depth according to the maximum codingunit.

Since the number of deeper coding units increases whenever the maximumcoding unit is split according to depths, encoding including theprediction encoding and the transformation is performed on all of thedeeper coding units generated as the depth deepens. For convenience ofdescription, the prediction encoding and the transformation will now bedescribed based on a coding unit of a current depth, in at least onemaximum coding unit.

The video encoding apparatus 100 may variously select a size or a shapeof a data unit for encoding the image data. To encode the image data,operations such as prediction encoding, transformation, and entropyencoding are performed, and at this time, the same data unit may be usedfor all the operations or different data units may be used for eachoperation.

For example, the video encoding apparatus 100 may select not only acoding unit for encoding the image data but also a data unit differentfrom the coding unit to perform the prediction encoding on the imagedata in the coding unit.

To perform prediction encoding in the maximum coding unit, theprediction encoding may be performed based on a coding unitcorresponding to a coded depth, i.e., based on a coding unit that is nolonger split into coding units corresponding to a lower depth.Hereinafter, the coding unit that is no longer split and becomes a basisunit for prediction encoding will now be referred to as a ‘predictionunit’. A partition obtained by splitting the prediction unit may includea prediction unit or a data unit obtained by splitting at least one of aheight and a width of the prediction unit.

For example, when a coding unit of 2N×2N (where N is a positive integer)is no longer split and becomes a prediction unit of 2N×2N, a size of apartition may be 2N×2N, 2N×N, N×2N, or N×N. Examples of a partition typemay include symmetrical partitions that are obtained by symmetricallysplitting a height or a width of the prediction unit, partitionsobtained by asymmetrically splitting the height or the width of theprediction unit, such as 1:n or n:1, partitions that are obtained bygeometrically splitting the prediction unit, and partitions havingarbitrary shapes.

A prediction mode of the prediction unit may be at least one of an intramode, an inter mode, and a skip mode. For example, the intra mode or theinter mode may be performed on the partition of 2N×2N, 2N×N, N×2N, orN×N. Also, the skip mode may be performed only on the partition of2N×2N. The encoding is independently performed on one prediction unit ina coding unit, thereby selecting a prediction mode having the leastencoding error.

The video encoding apparatus 100 may also perform the transformation onthe image data in a coding unit based not only on the coding unit forencoding the image data but also based on a data unit that is differentfrom the coding unit.

To perform the transformation in the coding unit, the transformation maybe performed based on a data unit having a size smaller than or equal tothe coding unit. For example, the data unit for the transformation mayinclude a data unit for an intra mode and a data unit for an inter mode.

A data unit used as a base of the transformation will now be referred toas a ‘transformation unit’. Similarly to the coding unit, thetransformation unit in the coding unit may be recursively split intosmaller sized regions so that the transformation unit may beindependently determined in units of regions. Thus, residual data in thecoding unit may be divided according to the transformation unit havingthe tree structure according to transformation depths.

A transformation depth indicating the number of times splitting isperformed to reach the transformation unit by splitting the height andthe width of the coding unit may also be set in the transformation unit.For example, in a current coding unit of 2N×2N, a transformation depthmay be 0 when a size of a transformation unit is 2N×2N, may be 1 whenthe size of the transformation unit is N×N, and may be 2 when the sizeof the transformation unit is (N/2)×(N/2). That is, the transformationunit having the tree structure may also be set according totransformation depths.

Encoding information according to coding units corresponding to a codeddepth requires not only information about the coded depth but also aboutinformation related to prediction encoding and transformation.Accordingly, the coding unit determiner 120 not only determines a codeddepth having the least encoding error but also determines a partitiontype in a prediction unit, a prediction mode according to predictionunits, and a size of a transformation unit for transformation.

Coding units according to a tree structure in a maximum coding unit anda method of determining a partition according to exemplary embodimentswill be described in detail later with reference to FIGS. 3 through 12.

The coding unit determiner 120 may measure an encoding error of deepercoding units according to depths by using rate-distortion optimization(R-DO) based on Lagrangian multipliers.

The output unit 130 outputs the image data of the maximum coding unit,which is encoded based on the at least one coded depth determined by thecoding unit determiner 120, and information about the encoding modeaccording to the coded depth in bit streams. The encoded image data maybe a coding result of residual data of an image. The information aboutthe encoding mode according to the coded depth may include informationabout the coded depth, information about the partition type in theprediction unit, prediction mode information, and size information ofthe transformation unit. In particular, as will be described later, whenentropy encoding a syntax element indicating a size of a transformationunit, the output unit 130 binarizes the syntax element indicating thesize of the transformation according to bit strings by using a parameterwhich is gradually updated. An operation of entropy encoding atransformation unit by using the output unit 130 will be describedlater.

The information about the coded depth may be defined by using splitinformation according to depths, which indicates whether encoding isperformed on coding units of a lower depth instead of a current depth.When the current depth of the current coding unit is the coded depth,image data in the current coding unit is encoded and output, and thusthe split information may be defined not to split the current codingunit to a lower depth. Alternatively, when the current depth of thecurrent coding unit is not the coded depth, the encoding is performed onthe coding unit of the lower depth, and thus the split information maybe defined to split the current coding unit to obtain the coding unitsof the lower depth.

When the current depth is not the coded depth, encoding is performed onthe coding unit that is split into the coding unit of the lower depth.Since at least one coding unit of the lower depth exists in one codingunit of the current depth, the encoding is performed on each coding unitof the lower depth, and thus the encoding may be recursively performedfor the coding units having the same depth.

Since the coding units having a tree structure are determined for onemaximum coding unit, and information about at least one encoding mode isdetermined for a coding unit of a coded depth, information about atleast one encoding mode may be determined for one maximum coding unit.Also, a coded depth of the image data of the maximum coding unit may bedifferent according to locations since the image data is hierarchicallysplit according to depths, and thus information about the coded depthand the encoding mode may be set for the image data.

Accordingly, the output unit 130 may assign encoding information about acorresponding coded depth and an encoding mode to at least one of thecoding unit, the prediction unit, and a minimum unit included in themaximum coding unit.

The minimum unit according to an exemplary embodiment may be asquare-shaped data unit obtained by splitting the minimum coding unitconstituting the lowermost depth by 4. Alternatively, the minimum unitmay be a maximum square-shaped data unit that may be included in all ofthe coding units, prediction units, partition units, and transformationunits included in the maximum coding unit.

For example, the encoding information output through the output unit 130may be classified into encoding information according to coding unitsand encoding information according to prediction units. The encodinginformation according to the coding units may include the informationabout the prediction mode and about the size of the partitions. Theencoding information according to the prediction units may includeinformation about an estimated direction of an inter mode, about areference image index of the inter mode, about a motion vector, about achroma component of an intra mode, and about an interpolation method ofthe intra mode. Also, information about a maximum size of the codingunit defined according to pictures, slices, or groups of pictures(GOPs), and information about a maximum depth may be inserted into aheader of a bit stream.

In the video encoding apparatus 100, the deeper coding unit may be acoding unit obtained by dividing a height or a width of a coding unit ofan upper depth, which is one layer above, by, for example, two. In otherwords, when the size of the coding unit of the current depth is 2N×2N,the size of the coding unit of the lower depth is N×N. Also, the codingunit of the current depth having the size of 2N×2N may include a maximumnumber of four coding units of the lower depth.

Accordingly, the video encoding apparatus 100 may form the coding unitshaving the tree structure by determining coding units having anappropriate (e.g., optimum) shape and an optimum size for each maximumcoding unit based on the size of the maximum coding unit and the maximumdepth determined considering characteristics of the current picture.Also, since encoding may be performed on each maximum coding unit byusing any one of various prediction modes and transformations, anoptimum encoding mode may be determined considering characteristics ofthe coding unit of various image sizes.

Thus, when an image having a higher resolution or a larger data amountis encoded in a related art macroblock, a number of macroblocks perpicture excessively increases. Accordingly, a number of pieces ofcompressed information generated for each macroblock increases, and thusit is difficult to transmit the compressed information and datacompression efficiency decreases. However, by using the video encodingapparatus 100 according to an exemplary embodiment, image compressionefficiency may be increased since a coding unit is adjusted consideringcharacteristics of an image while increasing a maximum size of a codingunit considering a size of the image.

FIG. 2 is a block diagram of a video decoding apparatus 200 according toan exemplary embodiment.

The video decoding apparatus 200 includes a receiver 210, an image dataand encoding information extractor 220, and an image data decoder 230.Definitions of various terms such as a coding unit, a depth, aprediction unit, a transformation unit, and information about variousencoding modes used for various operations of the video decodingapparatus 200 are substantially the same as those described withreference to FIG. 1 and the video encoding apparatus 100.

The receiver 210 receives a bit stream of an encoded video to parse asyntax element. The image data and encoding information extractor 220extracts syntax elements indicating encoded image data based on codingunits having a structure by performing entropy decoding of parsed syntaxelements, and outputs the extracted syntax elements to the image datadecoder 230. That is, the image data and encoding information extractor220 performs entropy decoding of syntax elements that are received in aform of bit strings of 0 and 1, thereby restoring the syntax elements.

Also, the image data and encoding information extractor 220 extractsinformation about a coded depth, an encoding mode, color componentinformation, prediction mode information, etc. for the coding unitshaving a tree structure according to each maximum coding unit from theparsed bitstream. The extracted information about the coded depth andthe encoding mode is output to the image data decoder 230. The imagedata in a bit stream is split into the maximum coding unit so that theimage data decoder 230 may decode the image data for each maximum codingunit.

The information about the coded depth and the encoding mode according tothe maximum coding unit may be set with respect to at least one codingunit corresponding to the coded depth, and information about an encodingmode may include information about a partition type, about a predictionmode, and a size of a transformation unit of a coding unit correspondingto the coded depth. Also, splitting information according to depths maybe extracted as the information about the coded depth.

The information about the coded depth and the encoding mode according toeach maximum coding unit extracted by the image data and encodinginformation extractor 220 is information about a coded depth and anencoding mode determined to generate a minimum encoding error determinedby an encoder, such as the video encoding apparatus 100, which performsencoding for each deeper coding unit according to respective depthsaccording to each maximum coding unit. Accordingly, the video decodingapparatus 200 may restore an image by decoding the image data accordingto a coded depth and an encoding mode that generates the minimumencoding error.

Since encoding information about the coded depth and the encoding modemay be assigned to a predetermined data unit from among a correspondingcoding unit, a prediction unit, and a minimum unit, the image data andencoding information extractor 220 may extract the information about thecoded depth and the encoding mode according to the predetermined dataunits. When information about a coded depth and encoding mode of acorresponding maximum coding unit is assigned to each of predetermineddata units, the predetermined data units to which the same informationabout the coded depth and the encoding mode is assigned may be inferredto be the data units included in the same maximum coding unit.

Also, as will be described later, the image data and encodinginformation extractor 220 de-binarizes a syntax element indicating asize of a transformation coefficient by using a parameter that isgradually updated. An operation of obtaining size information of atransformation coefficient by using the image data and encodinginformation extractor 220 by de-binarizing a bit string corresponding toa syntax element indicating a size of a transformation unit will bedescribed in detail later.

The image data decoder 230 restores the current picture by decoding theimage data in each maximum coding unit based on the information aboutthe coded depth and the encoding mode according to the maximum codingunits. In other words, the image data decoder 230 may decode the encodedimage data based on the extracted information about the partition type,the prediction mode, and the transformation unit for each coding unitfrom among the coding units having the tree structure included in eachmaximum coding unit. A decoding process may include prediction includingintra prediction and motion compensation, and inverse transformation.

The image data decoder 230 may perform intra prediction or motioncompensation according to a partition and a prediction mode of eachcoding unit, based on the information about the partition type and theprediction mode of the prediction unit of the coding unit according tocoded depths.

Also, the image data decoder 230 may perform inverse transformationaccording to each transformation unit in the coding unit based on theinformation about the size of the transformation unit of the coding unitaccording to coded depths to perform the inverse transformationaccording to maximum coding units.

The image data decoder 230 may determine at least one coded depth of acurrent maximum coding unit by using split information according todepths. When the split information indicates that image data is nolonger split in the current depth, the current depth is a coded depth.Accordingly, the image data decoder 230 may decode the coding unit ofthe current depth with respect to the image data of the current maximumcoding unit by using the information about the partition type of theprediction unit, the prediction mode, and the size of the transformationunit.

In other words, data units containing the encoding information includingthe same split information may be gathered by observing the encodinginformation set assigned for the predetermined data unit from among thecoding unit, the prediction unit, and the minimum unit, and the gathereddata units may be considered to be one data unit to be decoded by theimage data decoder 230 in the same encoding mode.

The video decoding apparatus 200 may obtain information about at leastone coding unit that generates the minimum encoding error when encodingis recursively performed for each maximum coding unit, and may use theinformation to decode the current picture. In other words, encoded imagedata of the coding units having the tree structure determined to be theoptimum coding units in each maximum coding unit may be decoded.

Accordingly, when image data have a higher resolution and a largeramount of data, the image data may be efficiently decoded and restoredby using a size of a coding unit and an encoding mode, which areadaptively determined according to characteristics of the image data byusing information about an optimum encoding mode received from anencoder.

A method of determining coding units having a tree structure, aprediction unit, and a transformation unit according to an exemplaryembodiment will now be described with reference to FIGS. 3 through 13.

FIG. 3 is a diagram for describing a concept of hierarchical codingunits according to an exemplary embodiment.

A size of a coding unit may be expressed in a format of width×height,and may be, for example, 64×64, 32×32, 16×16, and 8×8. A coding unit of64×64 may be split into partitions of 64×64, 64×32, 32×64, or 32×32; anda coding unit of 32×32 may be split into partitions of 32×32, 32×16,16×32, or 16×16; a coding unit of 16×16 may be split into partitions of16×16, 16×8, 8×16, or 8×8; and a coding unit of 8×8 may be split intopartitions of 8×8, 8×4, 4×8, or 4×4.

Regarding video data 310, a resolution of 1920×1080, a maximum size of acoding unit of 64, and a maximum depth of 2 are set. Regarding videodata 320, a resolution of 1920×1080, a maximum size of a coding unit of64, and a maximum depth of 3 are set. Regarding video data 330, aresolution of 352×288, a maximum size of a coding unit of 16, and amaximum depth of 1 are set. The maximum depth shown in FIG. 3 denotes atotal number of splits from a maximum coding unit to a minimum codingunit.

When a resolution is higher or a data amount is larger, a maximum sizeof a coding unit may be larger to not only increase encoding efficiencybut also to reflect characteristics of an image more accurately.Accordingly, the maximum size of the coding unit of the video data 310and 320 having a higher resolution than that of the video data 330 maybe set to 64, which is larger than the maximum size of the coding unitof the video data 330, i.e., 16.

Since the maximum depth of the video data 310 is 2, coding units 315 ofthe vide data 310 may include a maximum coding unit having a long axissize of 64 and coding units having long axis sizes of 32 and 16 sincedepths are deepened by two lower layers by splitting the maximum codingunit twice. On the other hand, since the maximum depth of the video data330 is 1, coding units 335 of the video data 330 may include a maximumcoding unit having a long axis size of 16 and coding units having a longaxis size of 8 since depths are deepened by one layer by splitting themaximum coding unit once.

Since the maximum depth of the video data 320 is 3, coding units 325 ofthe video data 320 may include a maximum coding unit having a long axissize of 64 and coding units having long axis sizes of 32, 16, and 8since the depths are deepened by 3 layers by splitting the maximumcoding unit three times. As a depth deepens, detailed information may beexpressed more precisely.

FIG. 4 is a block diagram of a video encoder 400 based on coding unitsaccording to an exemplary embodiment.

The video encoder 400 includes operations performed in the coding unitdeterminer 120 of the video encoding apparatus 100 to encode image data.That is, an intra predictor 410 performs intra prediction on codingunits in an intra mode, with respect to a current frame 405, and amotion estimator 420 and a motion compensator 425 respectively performinter estimation and motion compensation on coding units in an intermode by using the current frame 405 and a reference frame 495.

Data output from the intra predictor 410, the motion estimator 420, andthe motion compensator 425 are output as a quantized transformationcoefficient through a transformer 430 and a quantizer 440. The quantizedtransformation coefficient is restored as data in a spatial domainthrough an inverse quantizer 460 and an inverse transformer 470, and therestored data in the spatial domain is output as the reference frame 495after being post-processed through a deblocking unit 480 and a loopfiltering unit 490. The quantized transformation coefficient may beoutput as a bitstream 455 through an entropy encoder 450.

In order for the video encoder 400 to be applied to the video encodingapparatus 100, all the elements of the video encoder 400, i.e., theintra predictor 410, the motion estimator 420, the motion compensator425, the transformer 430, the quantizer 440, the entropy encoder 450,the inverse quantizer 460, the inverse transformer 470, the deblockingunit 480, and the loop filtering unit 490 need to perform operationsbased on each coding unit from among coding units having a treestructure while considering the maximum depth of each maximum codingunit.

Specifically, the intra predictor 410, the motion estimator 420, and themotion compensator 425 determine partitions and a prediction mode ofeach coding unit from among the coding units having a tree structurewhile considering the maximum size and the maximum depth of a currentmaximum coding unit, and the transformer 430 determines the size of thetransformation unit in each coding unit from among the coding unitshaving a tree structure.

FIG. 5 is a block diagram of a video decoder 500 based on coding unitsaccording to an exemplary embodiment.

A parser 510 parses encoded image data to be decoded and informationabout encoding required for decoding from a bitstream 505. The encodedimage data passes through an entropy decoder 520 and an inversequantizer 530 to be output as inversely quantized data. An inversetransformer 540 restores the inversely quantized data to image data in aspatial domain. An intra predictor 550 performs intra prediction oncoding units in an intra mode with respect to the image data in thespatial domain, and a motion compensator 560 performs motioncompensation on coding units in an inter mode by using a reference frame585.

The image data in the spatial domain, which passes through the intrapredictor 550 and the motion compensator 560, may be output as arestored frame 595 after being post-processed through a deblocking unit570 and a loop filtering unit 580. Also, the image data, which ispost-processed through the deblocking unit 570 and the loop filteringunit 580, may be output as the reference frame 585.

In order for the video decoder 500 to be applied to the video decodingapparatus 200, all the elements of the video decoder 500, i.e., theparser 510, the entropy decoder 520, the inverse quantizer 530, theinverse transformer 540, the intra predictor 550, the motion compensator560, the deblocking unit 570, and the loop filtering unit 580, performoperations based on coding units having a tree structure for eachmaximum coding unit.

The intra predictor 550 and the motion compensator 560 determine apartition and a prediction mode for each coding unit having a treestructure, and the inverse transformer 540 needs to determine a size ofa transformation unit for each coding unit.

FIG. 6 is a diagram illustrating deeper coding units according to depthsand partitions according to an exemplary embodiment.

The video encoding apparatus 100 and the video decoding apparatus 200use hierarchical coding units to consider characteristics of an image. Amaximum height, a maximum width, and a maximum depth of coding units maybe adaptively determined according to the characteristics of the imageor may be differently set by a user. Sizes of deeper coding unitsaccording to depths may be determined according to the predeterminedmaximum size of the coding unit.

In a hierarchical structure 600 of coding units according to anexemplary embodiment, the maximum height and the maximum width of thecoding units are 64, and the maximum depth is 4. Since a depth deepensalong a vertical axis of the hierarchical structure 600, a height and awidth of the deeper coding unit are split. Also, a prediction unit andpartitions, which are bases for prediction encoding of each deepercoding unit, are shown along a horizontal axis of the hierarchicalstructure 600.

In other words, a coding unit 610 is a maximum coding unit in thehierarchical structure 600, wherein a depth is 0 and a size, i.e., aheight by a width, is 64×64. The depth deepens along the vertical axis,and a coding unit 620 having a size of 32×32 and a depth of 1, a codingunit 630 having a size of 16×16 and a depth of 2, a coding unit 640having a size of 8×8 and a depth of 3, and a coding unit 650 having asize of 4×4 and a depth of 4 exist. The coding unit 650 having the sizeof 4×4 and the depth of 4 is a minimum coding unit.

The prediction unit and the partitions of a coding unit are arrangedalong the horizontal axis according to each depth. In other words, whenthe coding unit 610 having the size of 64×64 and the depth of 0 is aprediction unit, the prediction unit may be split into partitionsincluded in the coding unit 610, i.e. a partition 610 having the size of64×64, a partition 612 having a size of 64×32, a partition 614 having asize of 32×64, or a partition 616 having a size of 32×32, included inthe coding unit 610.

Similarly, a prediction unit of the coding unit 620 having the size of32×32 and the depth of 1 may be split into partitions included in thecoding unit 620, i.e. a partition 620 having the size of 32×32, apartition 622 having a size of 32×16, a partition 624 having a size of16×32, and a partition 626 having a size of 16×16, included in thecoding unit 620.

Similarly, a prediction unit of the coding unit 630 having the size of16×16 and the depth of 2 may be split into partitions included in thecoding unit 630, i.e. a partition having the size of 16×16, a partition632 having a size of 16×8, a partition 634 having a size of 8×16, and apartition 636 having a size of 8×8, included in the coding unit 630.

Similarly, a prediction unit of the coding unit 640 having the size of8×8 and the depth of 3 may be split into partitions included in thecoding unit 640, i.e. a partition having the size of 8×8, a partition642 having a size of 8×4, a partition 644 having a size of 4×8, and apartition 646 having a size of 4×4, included in the coding unit 640.

The coding unit 650 having the size of 4×4 and the depth of 4 is theminimum coding unit and a coding unit of the lowermost depth. Aprediction unit of the coding unit 650 is assigned to only a partitionhaving the size of 4×4.

To determine the at least one coded depth of the coding units includedin the maximum coding unit 610, the coding unit determiner 120 of thevideo encoding apparatus 100 performs encoding for coding unitscorresponding to each depth included in the maximum coding unit 610.

The number of deeper coding units according to depths including data inthe same range and the same size increases as the depth deepens. Forexample, four coding units corresponding to a depth of 2 are required tocover data that is included in one coding unit corresponding to a depthof 1. Accordingly, to compare encoding results of the same dataaccording to depths, the coding unit corresponding to the depth of 1 andfour coding units corresponding to the depth of 2 are each encoded.

To perform encoding for a current depth from among the depths, a leastencoding error may be selected for the current depth by performingencoding for each prediction unit in the coding units corresponding tothe current depth, along the horizontal axis of the hierarchicalstructure 600. Also, the minimum encoding error may be searched for bycomparing the least encoding errors according to depths and performingencoding for each depth as the depth deepens along the vertical axis ofthe hierarchical structure 600. A depth and a partition having theminimum encoding error in the maximum coding unit 610 may be selected asthe coded depth and a partition type of the maximum coding unit 610.

FIG. 7 is a diagram for describing a relationship between a coding unit710 and transformation units 720 according to an exemplary embodiment.

The video encoding apparatus 100 or the video decoding apparatus 200encodes or decodes an image according to coding units having sizessmaller than or equal to a maximum coding unit for each maximum codingunit. Sizes of transformation units for transformation during encodingmay be selected based on data units that are not larger than acorresponding coding unit.

For example, in the video encoding apparatus 100 or the video decodingapparatus 200, when a size of the coding unit 710 is 64×64,transformation may be performed by using the transformation units 720having a size of 32×32.

Also, data of the coding unit 710 having the size of 64×64 may beencoded by performing the transformation on each of the transformationunits having the size of 32×32, 16×16, 8×8, and 4×4, which are smallerthan 64×64, and then a transformation unit having the least coding errormay be selected from thereamong.

FIG. 8 is a diagram for describing encoding information of coding unitscorresponding to a coded depth according to an exemplary embodiment.

The output unit 130 of the video encoding apparatus 100 may encode andtransmit information 800 about a partition type, information 810 about aprediction mode, and information 820 about a size of a transformationunit for each coding unit corresponding to a coded depth as informationabout an encoding mode.

The information 800 indicates information about a shape of a partitionobtained by splitting a prediction unit of a current coding unit,wherein the partition is a data unit for prediction encoding the currentcoding unit. For example, referring to (a) of FIG. 8, a current codingunit CU_0 having a size of 2N×2N may be split into any one of apartition 802 having a size of 2N×2N, a partition 804 having a size of2N×N, a partition 806 having a size of N×2N, and a partition 808 havinga size of N×N. Here, the information 800 about a partition type is setto indicate one of the partition 802 having the size of 2N×2N, thepartition 804 having the size of 2N×N, the partition 806 having the sizeof N×2N, and the partition 808 having the size of N×N.

The information 810 indicates a prediction mode of each partition. Forexample, referring to (b) of FIG. 8, the information 810 may indicate amode of prediction encoding performed on a partition indicated by theinformation 800, i.e., an intra mode 812, an inter mode 814, or a skipmode 816.

The information 820 indicates a transformation unit to be based on whentransformation is performed on a current coding unit. For example,referring to (c) of FIG. 8, the transformation unit may be a first intratransformation unit 822, a second intra transformation unit 824, a firstinter transformation unit 826, or a second inter transformation unit828.

The image data and encoding data extracting unit 210 of the videodecoding apparatus 200 may extract and use, for decoding, theinformation 800 about coding units, the information 810 about aprediction mode, and the information 820 about a size of atransformation unit according to each deeper coding unit.

FIG. 9 is a diagram of deeper coding units according to depths accordingto an exemplary embodiment.

Split information may be used to indicate a change of a depth. The spiltinformation indicates whether a coding unit of a current depth is splitinto coding units of a lower depth.

A prediction unit 910 for prediction encoding of a coding unit CU_0 900having a depth of 0 and a size of 2N_0×2N_0 may include partitions of apartition type 912 having a size of 2N_0×2N_0, a partition type 914having a size of 2N_0×N_0, a partition type 916 having a size ofN_0×2N_0, and a partition type 918 having a size of N_0×N_0. FIG. 9 onlyillustrates the partition types 912 through 918 which are obtained bysymmetrically splitting the prediction unit 910, but a partition type isnot limited thereto. For example, the partitions of the prediction unit910 may include asymmetrical partitions, partitions having apredetermined shape, and partitions having a geometrical shape.

Prediction encoding is respectively performed on one partition having asize of 2N_0×2N_0, two partitions having a size of 2N_0×N_0, twopartitions having a size of N_0×2N_0, and four partitions having a sizeof N_0×N_0, according to each partition type. The prediction encoding inan intra mode and an inter mode may be performed on the partitionshaving the sizes of 2N_0×2N_0, N_(—0×)2N_0, 2N_0×N_0, and N_0×N_0. Theprediction encoding in a skip mode is performed only on the partitionhaving the size of 2N_0×2N_0.

When an encoding error is the smallest in one of the partition types 912through 916 having the sizes of 2N_0×2N_0, 2N_0×N_0, and N_0×2N_0, theprediction unit 910 may not be split into a lower depth.

When the encoding error is the smallest in the partition type 918 havingthe size of N_0×N_0, a depth is changed from 0 to 1 to split thepartition type 918 in operation 920, and encoding is respectivelyperformed on coding units CU_1 930 having a depth of 1 and a size ofN_0×N_0 to search for a minimum encoding error.

A prediction unit 940 for prediction encoding of a coding unit 930having a depth of 1 and a size of 2N_1×2N_1 (i.e., N_0×N_0) may includepartitions of a partition type 942 having a size of 2N_1×2N_1, apartition type 944 having a size of 2N_1×N_1, a partition type 946having a size of N_1×2N_1, and a partition type 948 having a size ofN_1×N_1.

When an encoding error is the smallest in the partition type 948 havingthe size of N_1×N_1, a depth is changed from 1 to 2 to split thepartition type 948 in operation 950, and encoding is respectivelyperformed on coding units 960, which have a depth of 2 and a size ofN_2×N_2 to search for a minimum encoding error.

When a maximum depth is d, a split operation according to each depth maybe performed up to when a depth becomes d-1, and split information maybe encoded up to when a depth is one of 0 to d-2. In other words, whenencoding is performed up to when the depth is d-1 after a coding unitcorresponding to a depth of d-2 is split in operation 970, a predictionunit 990 for prediction encoding a coding unit CU_(d-1) 980 having adepth of d-1 and a size of 2N_(d-1)×2N_(d-1) may include partitions of apartition type 992 having a size of 2N_(d-1)×2N_(d-1), a partition type994 having a size of 2N_(d-1)×N_(d-1), a partition type 996 having asize of N_(d-1)×2N_(d-1), and a partition type 998 having a size ofN_(d-1)×N_(d-1).

Prediction encoding may be respectively performed on one partition 992having the size of 2N_(d-1)×2N_(d-1), two partitions 994 having the sizeof 2N_(d-1)×N_(d-1), two partitions 996 having the size ofN_(d-1)×2N_(d-1), four partitions 998 having the size of N_(d-1)×N_(d-1)to search for a partition type having a minimum encoding error fromthereamong.

When the partition type 998 having the size of N_(d-1)×N_(d-1) has theminimum encoding error, since a maximum depth is d, the coding unitCU_(d-1) 980 having a depth of d-1 is no longer split to a lower depth,and a coded depth for the coding units included in the current maximumcoding unit 900 is determined to be (d-1) and a partition type of thecurrent maximum coding unit 900 may be determined to be N_(d-1)×N_(d-1).Also, since the maximum depth is d, split information for the codingunit 980 is not set.

A data unit 999 may be a ‘minimum unit’ for the current maximum codingunit. A minimum unit according to an exemplary embodiment may be arectangular data unit obtained by splitting the minimum coding unit 980by 4. By performing the encoding repeatedly, the video encodingapparatus 100 may select a depth having the least encoding error bycomparing encoding errors according to depths of the coding unit 900 todetermine a coded depth, and set a corresponding partition type and aprediction mode as an encoding mode of the coded depth.

As such, the minimum encoding errors according to depths are comparedwith respect to all of the depths of 1 through d, and a depth having theleast encoding error may be determined as a coded depth. The codeddepth, the partition type of the prediction unit, and the predictionmode may be encoded and transmitted as information about an encodingmode. Also, since a coding unit is split from a depth of 0 to a codeddepth, only split information of the coded depth is set to 0, and splitinformation of depths excluding the coded depth is set to 1.

The image data and encoding information extractor 220 of the videodecoding apparatus 200 may extract and use the information about thecoded depth and the prediction unit of the coding unit 900 to decode thecoding unit 912. The video decoding apparatus 200 may determine a depth,in which split information is 0, as a coded depth by using splitinformation according to depths, and use information about an encodingmode of the corresponding depth for decoding.

FIGS. 10 through 12 are diagrams for describing a relationship betweencoding units 1010, prediction units 1060, and transformation units 1070according to an exemplary embodiment.

The coding units 1010 are coding units having a tree structure,corresponding to coded depths determined by the video encoding apparatus100, in a maximum coding unit. The prediction units 1060 are partitionsof prediction units of each of the coding units 1010, and thetransformation units 1070 are transformation units of each of the codingunits 1010.

When a depth of a maximum coding unit is 0 in the coding units 1010,depths of coding units 1012 and 1054 are 1, depths of coding units 1014,1016, 1018, 1028, 1050, and 1052 are 2, depths of coding units 1020,1022, 1024, 1026, 1030, 1032, and 1048 are 3, and depths of coding units1040, 1042, 1044, and 1046 are 4.

In the prediction units 1060, some coding units 1014, 1016, 1022, 1032,1048, 1050, 1052, and 1054 are obtained by splitting the coding units.In other words, partition types in the coding units 1014, 1022, 1050,and 1054 have a size in a form of 2N×N, partition types in the codingunits 1016, 1048, and 1052 have a size in a form of N×2N, and apartition type of the coding unit 1032 has a size in a form of N×N.Prediction units and partitions of the coding units 1010 are smallerthan or equal to each coding unit.

Transformation or inverse transformation is performed on image data ofthe coding unit 1052 in the transformation units 1070 in a data unitthat is smaller than the coding unit 1052. Also, the coding units 1014,1016, 1022, 1032, 1048, 1050, 1052, and 1054 in the transformation units1070 are different from those in the prediction units 1060 in terms ofsizes and shapes. In other words, the video encoding apparatus 100 andthe video decoding apparatus 200 may perform intra prediction, motionestimation, motion compensation, transformation, and inversetransformation individually on a data unit in the same coding unit.

Accordingly, encoding is recursively performed on each of coding unitshaving a hierarchical structure in each region of a maximum coding unitto determine an optimum coding unit, and thus coding units having arecursive tree structure may be obtained. Encoding information mayinclude split information about a coding unit, information about apartition type, information about a prediction mode, and informationabout a size of a transformation unit.

Table 1 shows the encoding information that may be set by the videoencoding apparatus 100 and the video decoding apparatus 200.

TABLE 1 Split Information 0 (Encoding on Coding Unit having Size of 2N ×2N and Current Depth of d) Size of Transformation Unit Split SplitPartition Type Information 0 Information 1 Symmetrical Asymmetrical ofof Prediction Partition Partition Transformation Transformation SplitMode Type Type Unit Unit Information 1 Intra 2N × 2N 2N × nU 2N × 2N N ×N Respectively Inter 2N × N 2N × nD (Symmetrical Encode Skip N × 2N nL ×2N Partition Type) Coding Units (Only for N × N nR × 2N N/2 × N/2 Having2N × 2N) (Asymmetrical Lower Depth Partition Type) of d + 1

The output unit 130 of the video encoding apparatus 100 may output theencoding information about the coding units having a tree structure, andthe image data and encoding information extractor 220 of the videodecoding apparatus 200 may extract the encoding information about thecoding units having a tree structure from a received bitstream.

Split information indicates whether a current coding unit is split intocoding units of a lower depth. When the split information of a currentdepth d is 0, a depth, in which a current coding unit is no longer splitinto a lower depth, is a coded depth, and thus information about apartition type, a prediction mode, and a size of a transformation unitmay be defined for the coded depth. When the current coding unit isfurther split according to the split information, encoding isindependently performed on four split coding units of a lower depth.

A prediction mode may be one of an intra mode, an inter mode, and a skipmode. The intra mode and the inter mode may be defined in all partitiontypes, and the skip mode is defined only in a partition type having asize of 2N×2N.

The information about the partition type may indicate symmetricalpartition types having sizes of 2N×2N, 2N×N, N×2N, and N×N, which areobtained by symmetrically splitting a height or a width of a predictionunit, and asymmetrical partition types having sizes of 2N×nU, 2N×nD,nL×2N, and nR×2N, which are obtained by asymmetrically splitting theheight or the width of the prediction unit. The asymmetrical partitiontypes having the sizes of 2N×nU and 2N×nD may be respectively obtainedby splitting the height of the prediction unit in 1:n and n:1 (where nis an integer greater than 1), and the asymmetrical partition typeshaving the sizes of nL×2N and nR×2N may be respectively obtained bysplitting the width of the prediction unit in 1:n and n:1.

The size of the transformation unit may be set to be two types in theintra mode and two types in the inter mode. In other words, when splitinformation of the transformation unit is 0, the size of thetransformation unit may be 2N×2N, which is the size of the currentcoding unit. When the split information of the transformation unit is 1,the transformation units may be obtained by splitting the current codingunit. Also, when a partition type of the current coding unit having thesize of 2N×2N is a symmetrical partition type, a size of atransformation unit may be N×N, and when the partition type of thecurrent coding unit is an asymmetrical partition type, the size of thetransformation unit may be N/2×N/2.

The encoding information about coding units having a tree structure mayinclude at least one of a coding unit corresponding to a coded depth, aprediction unit, and a minimum unit. The coding unit corresponding tothe coded depth may include at least one of a prediction unit and aminimum unit containing the same encoding information.

Accordingly, it is determined whether adjacent data units are includedin the same coding unit corresponding to the coded depth by comparingencoding information of the adjacent data units. Also, a correspondingcoding unit corresponding to a coded depth is determined by usingencoding information of a data unit, and thus a distribution of codeddepths in a maximum coding unit may be determined.

Accordingly, when a current coding unit is predicted based on encodinginformation of adjacent data units, encoding information of data unitsin deeper coding units adjacent to the current coding unit may bedirectly referred to and used.

Alternatively, when a current coding unit is predicted based on encodinginformation of adjacent data units, data units adjacent to the currentcoding unit are searched using encoded information of the data units,and the searched adjacent coding units may be referred to for predictingthe current coding unit.

FIG. 13 is a diagram for describing a relationship between a coding unit(CU), a prediction unit (PU), and a transformation unit (TU) accordingto the encoding mode information of Table 1.

A maximum coding unit 1300 includes coding units 1302, 1304, 1306, 1312,1314, 1316, and 1318 of coded depths. Here, since the coding unit 1318is a coding unit of a coded depth, split information may be set to 0.Information about a partition type of the coding unit 1318 having a sizeof 2N×2N may be set to be one of a partition type 1322 having a size of2N×2N, a partition type 1324 having a size of 2N×N, a partition type1326 having a size of N×2N, a partition type 1328 having a size of N×N,a partition type 1332 having a size of 2N×nU, a partition type 1334having a size of 2N×nD, a partition type 1336 having a size of nL×2N,and a partition type 1338 having a size of nR×2N.

When the partition type is set to be symmetrical, i.e. the partitiontype 1322, 1324, 1326, or 1328, a transformation unit 1342 having a sizeof 2N×2N is set when split information (or transformation unit (TU) sizeflag) of a transformation unit is 0, and a transformation unit 1344having a size of N×N is set when the TU size flag is 1.

When the partition type is set to be asymmetrical, i.e., the partitiontype 1332, 1334, 1336, or 1338, a transformation unit 1352 having a sizeof 2N×2N is set when the TU size flag is 0, and a transformation unit1354 having a size of N/2×N/2 is set when the TU size flag is 1.

Hereinafter, motion prediction and compensation performed in the motionestimator 420 and the motion compensator 425 of the image encodingapparatus 100 of FIG. 4 and the motion compensator 560 of the imagedecoding apparatus 500 of FIG. 5 and encoding and decoding of motionprediction information performed in the entropy encoder 450 of FIG. 4and the entropy decoder 520 of FIG. 5 will be described in detail. Inthe description below, the above-described prediction unit may bereferred to as a block.

The motion estimator 420 generates a prediction value by performingunidirectional prediction on a prediction unit included in a P slice.Also, the motion estimator 420 generates a prediction value byperforming unidirectional prediction or bidirectional prediction on aprediction unit included in a B slice, by using a reference pictureincluded in two lists, List 0 and List 1 (not shown). According to therelated art, a reference picture is limited to a sheet of a referencepicture that immediately precedes a current picture and a sheet of areference picture that follows the current picture in bidirectionalprediction performed in MPEG-2. On the other hand, in the bidirectionalprediction mode performed by the motion estimator 420 according to anexemplary embodiment, any two sheets of reference pictures may be used,without being limited to previous and subsequent reference pictures of acurrent picture, and this mode may be referred to as a bi-predictivemode.

In a motion prediction mode of a current prediction unit, a cost of aresult of encoding a prediction value obtained by performingunidirectional prediction with respect to the current prediction unit byreferring to reference pictures included in a combination reference listand a cost of a result of encoding a prediction value obtained byperforming bidirectional motion prediction on the current predictionunit by using a first reference picture (e.g., L0 picture) included in afirst reference picture list List 0 (not shown) and a second referencepicture (e.g., L1 picture) included in a second reference picture listList 1 (not shown) may be compared, and a prediction mode having asmaller cost may be determined as a final prediction mode of the currentprediction unit. When comparing the costs, a prediction mode having ahigher efficiency may be determined based on, for example,rate-distortion.

FIG. 14 is a diagram illustrating an example of a reference picturereferred to by a prediction unit in a B picture according to anexemplary embodiment.

Referring to FIG. 14, a reference picture A 1430 and a reference pictureB 1420 are forward pictures having picture order counts (POCs) thatprecede to that of a current picture 1410, and a reference picture C1440 and a reference picture D 1450 are backward pictures having POCsthat follow that of the current picture 1410.

When the current picture 1410 is a B picture, prediction units in thecurrent picture 1410 are encoded based on one prediction mode among anintra prediction mode, a unidirectional prediction mode, a bidirectionalprediction mode, and a direct prediction mode. In an intra predictionmode, a current prediction unit is predicted by using values of adjacentpixels. In a unidirectional prediction mode, a current prediction unitis predicted by using a reference picture in a combination referencepicture list which is a combination of a first reference picture listList 0 and a second reference picture list List 1. In a bidirectionalprediction mode, a current prediction unit is predicted by using a totalof two reference pictures, that is, a first reference picture of a firstreference picture list List 0 and a second reference picture of a secondreference picture list List 1. In a direct prediction mode, a predictionmotion vector generated by using a motion vector of an adjacentprediction unit of a current prediction unit is used as a motion vectorof a current prediction unit, and in this mode, only prediction modeinformation and residual information are used as encoding informationfor encoding.

A prediction unit in a B picture that is unidirectoinally orbidirectionally predicted may be classified as: i) a prediction unitpredicted by referring to two different reference pictures in the samedirection; ii) a prediction unit predicted by referring to two differentreference pictures in different directions; iii) a prediction unitpredicted by referring to a single reference picture twice; and iv) aprediction unit predicted by referring to any single reference pictureonce. The prediction unit of i) through iii) correspond to a predictionunit predicted bidirectionally by referring to two reference pictures,and the prediction unit of iv) corresponds to a prediction unit that ispredicted unidirectionally by using a single reference picture.

As an example of i), a prediction unit 1411 is predicted by using anaverage of a corresponding block 1431 of the reference picture A 1430preceding the current picture 1410 and a corresponding block 1421 of thereference picture B 1420 also preceding the current picture 1410. As anexample of ii), a prediction unit 1413 is predicted by using an averageof a corresponding block 1423 of the reference picture B 1420 and acorresponding block 1441 of a reference picture C 1440. As an example ofiii), a prediction unit 1414 is predicted by using an average ofcorresponding blocks 1432 and 1433 of the reference picture A 1430. Asan example of iv), a prediction unit 1412 is predicted by using acorresponding block 1422 of the reference picture B 1420 and aprediction unit 1415 is predicted by using a corresponding block 1451 ofa reference picture D 1450.

As described above, the motion estimator 420 generates a predictionvalue of a prediction unit by performing inter-prediction by using aplurality of reference pictures for each prediction unit. In order for adecoder's end to generate a prediction value of a prediction unit,reference picture information and prediction direction information thatindicate which picture is referred to by each prediction unit, that is,prediction mode information, needs to be transmitted.

FIGS. 15A through 15C illustrate examples of a reference picture indexallocated to a reference picture according to an exemplary embodiment.

The motion estimator 420 uses, as a reference picture of a predictionunit that is bidirectionally predicted, two reference pictures, that is,a first reference picture of a first reference picture list List 0 and asecond reference picture of a second reference picture list List 1. Areference picture index L0_idx indicating each reference picture in thefirst reference picture list List 0 is allocated such that, asillustrated in FIG. 15A, a smaller reference picture index of forwardpictures 1520 is allocated to a picture closer to a current picture1510, and a smaller reference picture index of backward pictures 1530 isallocated to a picture closer to the current picture 1510. A referencepicture index L1_idx indicating each reference picture in the secondreference picture list List 1 is allocated such that, as illustrated inFIG. 15A, a smaller reference picture index of the backward pictures1530 is allocated to a picture closer to the current picture 1510, and asmaller reference picture index of the forward pictures 1520 isallocated to a picture closer to the current picture 1510.

Referring to FIGS. 15A and 15B, in the first reference picture list List0, a reference picture index L0_idx is allocated in an order from a mostrecent past picture Ref 2, a previous picture thereto Ref 1, a closestfuture picture Ref 4, to a next subsequent picture thereto Ref 5.Referring to FIGS. 15A and 15C, in the second reference picture listList 1, a reference picture index L1_idx is allocated in an order fromthe closest future picture Ref 4, the next subsequent picture theretoRef 5, the most recent past picture Ref 2, to a previous picture theretoRef 1.

As will be described later, instead of encoding a first referencepicture index L0_idx indicating a first reference picture in the firstreference picture list List 0 and a second reference picture indexL1_idx indicating a second reference picture in the second referencepicture list List 1 without any change, the entropy encoder 450 encodesa single reference syntax (Ref Syntax) indicating a motion predictionmode and a reference picture that are used in encoding a currentprediction unit based on a number of possible cases of a unidirectionalmotion prediction mode and a number of possible cases of a bidirectionalmotion prediction mode.

Reference picture information of a prediction unit that isunidirectionally predicted by the motion estimator 420 also needs to betransmitted to a decoder's end. Reference picture information indicatinga reference picture used in unidirectional prediction may be transmittedby using a reference syntax allocated to reference pictures in acombination reference picture list which is a combination of the firstreference picture list List 0 and a second reference picture list List1.

FIGS. 16A and 16B illustrate a combination reference picture list usedin unidirectional prediction, according to an exemplary embodiment.

The entropy encoder 450 may generate a combination reference picturelist which is a combination of a first reference picture list List 0 anda second reference picture list List 1 used in bidirectional prediction,allocate a reference syntax (Ref Syntax) to each of reference picturesincluded in the combination reference picture list, and encodeunidirectional prediction mode information and reference pictureinformation used in the unidirectional prediction mode by using theallocated reference syntax (Ref Syntax).

For example, referring to FIG. 16A, the entropy encoder 450 maysequentially scan reference pictures of the first reference picture listList 0 1610 and the second reference picture list List 1 1620 in anarrow direction, include newly scanned reference pictures in thecombination reference picture list, and exclude previously scannedreference pictures from the combination reference picture list, therebygenerating the combination reference picture list 1630. In thecombination reference picture list, a reference picture Ref 4 1611 ofthe first reference picture list List 0 1610, which is previouslyscanned, and a reference picture Ref 2 1621 of the second referencepicture list List 1 1620, which is previously scanned, are overlappedwith previously scanned reference pictures, and thus are not added tothe combination reference picture list 1630. When a current predictionunit is unidirectionally motion predicted by using a reference pictureon the combination reference picture list, the entropy encoder 450encodes reference syntax information allocated to the reference pictureused in unidirectional prediction as motion prediction information of acurrent prediction unit. For example, when the current prediction unitis unidirectionally predicted by using the reference picture Ref 4, theentropy encoder 450 encodes a reference syntax (Ref Syntax) having avalue of 1 with respect to a prediction mode and reference pictureinformation of the current prediction unit. When the reference syntax(Ref Syntax) having a value of 1 is received at a decoder's end, it maybe determined that the current prediction unit is unidirectionallypredicted by referring to the reference picture Ref 4.

In addition to the method as illustrated in FIG. 16A described above,the combination reference picture list may also be generated usingvarious methods in which only different reference pictures are included,except repeated reference pictures of the first reference picture listand the second reference picture list. For example, referring to FIG.16B, the entropy encoder 450 may generate a combination referencepicture list 1650 by removing repeated reference pictures 1641 and 1642from a reference picture list 1640 that is generated by sequentiallyscanning reference pictures of a first reference picture list List 0 anda second reference picture list List 1. A method of generating acombination reference picture list may be preferably set to be the sameat both an encoder's end and an decoder's end. When a plurality ofcombination reference picture lists are generated by using variousmethods, a predetermined index may be allocated to each method ofgenerating a combination reference picture list, and an index of amethod of generating, that is used in generating a combination referencelist at the encoder's end may be additionally transmitted to thedecoder's end. When methods of generating a combination referencepicture list are preset at the encoder's and decoder's ends, an indexabout a method of generating a combination reference picture list asdescribed above does not need to be transmitted.

The entropy encoder 450 encodes information about the unidirectionalprediction mode, the bidirectional prediction mode, and referencepictures according to each prediction mode, by using a single referencesyntax (Ref Syntax).

As described above, when a current prediction unit is unidirectionallypredicted by using a reference picture on the combination referencepicture list, the entropy encoder 450 encodes reference syntaxinformation allocated to the reference picture used in unidirectionalprediction as motion prediction information of the current predictionunit. That is, in the exemplary embodiment of FIG. 16A described above,when a reference syntax (Ref Syntax) has a value of 0, it indicates thatthe current prediction unit is unidirectionally predicted by referringto a reference picture Ref 2; when the reference syntax (Ref Syntax) hasa value of 1, it indicates that the current prediction unit isunidirectionally predicted by referring to a reference picture Ref 4;when the reference syntax has a value of 2, it indicates that thecurrent prediction unit is unidirectionally predicted by referring to areference picture Ref 1; and when the reference syntax (Ref Syntax) hasa value of 3, it indicates that the current prediction unit isunidirectionally predicted by referring to a reference picture Ref 5.

To encode a bidirectional prediction mode and information about tworeference pictures L0 picture and L1 picture used in the bidirectionalprediction mode in a manner different from the unidirectional predictionmode, the entropy encoder 450 allocates a reference syntax to eachcombination of reference pictures available in a bidirectionalprediction mode based on a number of cases of a unidirectional motionprediction mode and a number of cases of the bidirectional motionprediction mode, and encodes the reference syntax value allocated to acombination of reference pictures used in bidirectional prediction of acurrent prediction unit as motion prediction information.

FIG. 17 illustrates a table showing reference syntax values (Value)indicating unidirectional and bidirectional motion prediction modes andreference pictures allocated based on a number of cases ofunidirectional motion prediction modes and a number of cases ofbidirectional motion prediction modes according to an exemplaryembodiment.

Referring to FIG. 17, MaxVal is determined based on a sum (MaxValue) ofthe number of cases of the unidirectional motion prediction mode and thenumber of cases of the bidirectional motion prediction mode. MaxValrefers to a maximum value of a reference syntax value (Value), andMaxVal=MaxValue−1.

As described above, in a unidirectional motion prediction mode, thenumber of cases is classified according to which reference picture ofthe combination reference picture list is referred to, and consequently,the number of cases of a unidirectional motion prediction mode isdetermined by a number of reference pictures NumOfRef_LC in acombination reference picture list.

When a number of reference pictures included in a first referencepicture list is NumOfRef_L0, a number of reference pictures included ina second reference picture list is NumOfRef_L1, and a number ofreference pictures that are repeatedly included in the first and secondreference picture lists is NumOfRedundancy, the number of referencepictures NumOfRef_LC included in the combination reference picture listmay be NumOfRef_L0+NumOfRef_L1−NumOfRedundancy, indicating the number ofunrepeated reference pictures included in the combination referencepicture list.

Instead of setting a reference syntax for every possible case of aunidirectional motion prediction mode, a reference syntax as illustratedin FIG. 17 may be allocated up to a number of MaxCombinedRefNum, andwhen the number of possible cases exceeds MaxCombinedRefNum, anadditional process may be performed. For example, when MaxCombinedRefNumis 4, the entropy encoder 450 may allocate a reference syntax only tofirst through fourth reference pictures included in the combinationreference picture list, and when a reference picture of aunidirectionally predicted current prediction unit is included in areference picture on a combination reference picture list, the entropyencoder 450 determines a single reference syntax indicating aunidirectional prediction mode of a current prediction unit and areference picture that is used therein and encodes the reference syntax.When a reference picture referred to by a current prediction unit is notpredefined in a reference syntax allocation table, a reference syntaxvalue may be encoded to MaxValue. In other words, when a referencesyntax has a value MaxValue, this may be an exceptional case from thoseaccording to prediction modes and reference pictures of FIG. 17, andhere, an additional, exceptional process may be performed. A predictionmode and reference picture information may be additionally encoded withrespect to a current prediction unit of the exceptional process.

When a maximum number of reference pictures in a combination referencepicture list that is preset by the reference syntax allocation table islimited to be less than or equal to MaxCombinedRefNum, the number ofreference pictures NumOfRef_LC included in the combination referencepicture list is min(MaxCombinedRefNum,NumOfRef_L0+NumOfRef_L1−NumOfRedundancy). When the number of referencepictures included in a first reference picture list and a secondreference picture list that are preset by the reference syntaxallocation table is limited to a predetermined number n (where n is aninteger), NumOfRef_L0 is controlled to be min(n, NumOfRef_L0), andNumOfRef_L1 is controlled to be min(n, NumOfRef_L1).

A number of cases of a bidirectional motion prediction mode isclassified according to which reference picture of a first referencepicture list List 0 is used as a first reference picture and whichreference picture of a second reference picture list List 1 is used as asecond reference picture. Consequently, a number of cases of abidirectional motion prediction mode has a value ofNumOfRef_L0*NumOfRef_L1. For example, when two reference pictures areincluded in the first reference picture list List 0, and two referencepictures are included in the second reference picture list List 1, anumber of cases of a bidirectional motion prediction mode is 2*2, thatis, a total of four cases.

Therefore, a sum (MaxValue) of a number of cases of a unidirectionalmotion prediction mode and a number of cases of a bidirectional motionprediction mode has a value according to the following equation:MaxValue=NumOfRef_LC+NumOfRef_L0*NumOfRef_L1.

Accordingly, the entropy encoder 450 allocates a value from 0 to(MaxValue−1) to reference pictures used in a unidirectional motionprediction mode and a combination of reference pictures used in abidirectional motion prediction mode, and encodes the allocated value asa value of a reference syntax (Value), thereby encoding informationabout a motion prediction mode and reference picture information used inthe motion prediction, by using a single syntax.

Hereinafter, an operation of adaptively determining a reference syntaxaccording to a number of reference pictures included in the firstreference picture list List 0 and the second reference picture list List1 and a number of cases included in a combination reference picture listwill be described in detail with reference to FIG. 17. The maximumnumber of reference pictures to be included in each of the firstreference picture list List 0 and the second reference picture list List1 will be assumed to be two. Also, a motion prediction mode InterDir inFIG. 17 denotes a motion prediction direction, LC denotes aunidirectional motion prediction mode that uses a combination referencepicture list, and BI denotes a bidirectional motion prediction mode inwhich a first reference picture L0 picture of the first referencepicture list List 0 and a second reference picture L1 picture of thesecond reference picture list List 1 are used. In addition, a referencepicture index RefIdx in FIG. 17 indicates a reference picture used in aunidirectional motion prediction mode or in a bidirectional motionprediction mode, and RefIdx in the unidirectional motion prediction modeLC is a value indicating a reference picture on a combination referencepicture list like the reference syntax Ref Syntax described above withreference to FIG. 16A. Regarding (x,y) (x and y are 0 or 1), which isRefIdx in the bidirectional motion prediction mode BI, an x valueindicates a reference picture index of the first reference picture L0picture of the first reference picture list List 0, and a y valueindicates a reference picture index of the second reference picture L1picture of the second reference picture list List 1. For example, when(RefIdx)=(0, 0), this indicates a bidirectional motion prediction modewhere a first reference picture L0 picture having a value of RefIdx=0 ofthe first reference picture list List 0 and a second reference pictureL1 picture having a value of RefIdx=0 of the second reference picturelist List 1 are used.

a) When MaxVal=1 (Reference Numeral 1710)

When the first reference picture list List 0 and the second referencepicture list List 1 each include only one sheet of a reference picture(L0=1, L1=1), and the L0 picture and the L1 picture are identical, onlyone sheet of the reference picture is included in a combinationreference picture list. Thus, only one case of unidirectional motionprediction that is performed by using only one sheet of the referencepicture exists, and only one case of bidirectional motion predictionthat is performed using the one sheet of the reference picture (L0, L1)twice exists.

Accordingly, the entropy encoder 1410 allocates 0 as a value of areference syntax (Ref Syntax) when unidirectional motion prediction isperformed; and the entropy encoder 1410 allocates 1 as a value of areference syntax (Ref Syntax) when bidirectional motion prediction isperformed, and 0 or 1 is encoded as motion information of a currentprediction unit according to a prediction mode applied to the currentprediction unit.

At a decoder's end, when the first reference picture list List 0 and thesecond reference picture list List 1 each include only one sheet ofreference picture (L0=1, L1=1), the L0 picture and the L1 picture areidentical, and 0 is received as a reference syntax (Ref Syntax),unidirectional motion prediction is performed using the L0 picture (orL1 picture), and when 1 is received, bidirectional motion prediction isperformed by referring to the L0 picture (or L1 picture) twice.

b) When MaxVal=2 (Reference Numeral 1720)

When the first reference picture list List 0 and the second referencepicture list List 1 each include only one sheet of a reference picture(L0=1, L1=1), and the L0 picture and the L1 picture are not identical,and the L0 picture and the L1 picture are not identical, a total of twosheets of reference pictures are included in a combination referencepicture list and thus, two cases of unidirectional motion predictionwhere the two sheets of reference pictures are used exist. In addition,only one case of bidirectional motion prediction that is performed byreferring to each reference picture L0 and L1 pictures of the firstreference picture list List 0 and the second reference picture list List1 exists.

Accordingly, the entropy encoder 1410 allocates 0 or 1 as a value of areference syntax (Ref Syntax) according to which reference picture isused during unidirectional motion prediction of a prediction unit of acurrent picture, and when bidirectional motion prediction is performed,2 is allocated as a value of a reference syntax (Ref Syntax), and one of0 through 2 is encoded, as motion information, according to a predictionmode and a reference picture applied to the current prediction unit.

At a decoder's end, when the first reference picture list List 0 and thesecond reference picture list List 1 each include only one sheet of areference picture (L0=1, L1=1), the L0 picture and the L1 picture arenot identical, and 0 is received as a reference syntax (Ref Syntax),unidirectional motion prediction is performed using a first referencepicture of the two reference pictures included in the combinationreference picture list, and when 1 is received as a reference syntax(Ref Syntax), unidirectional motion prediction is performed using asecond reference picture of the two reference pictures included in thecombination reference picture list. Also, at the decoder's end, when 2is received as a reference syntax (Ref Syntax), bidirectional motionprediction is performed by referring to the L0 picture and the L1picture.

c) When MaxVal=3 (Reference Numeral 1730)

When the first reference picture list List 0 includes two referencepictures, and the second reference picture list List 1 includes only onesheet of a reference picture (L0=2, L1=1), and one of L0 picture and L1picture is overlapped, a total of two sheets of reference pictures areincluded in a combination reference picture list. Accordingly, two casesof unidirectional motion prediction where two sheets of referencepictures are used exist. In addition, two cases ((0,0), (1,0)) ofbidirectional motion prediction that is performed by referring to eachreference picture (L0 and L1 pictures) of the first reference picturelist List 0 and the second reference picture list List 1 exist.

Accordingly, the entropy encoder 1410 allocates 0 or 1 as a value of areference syntax (Ref Syntax) according to which reference picture isused during unidirectional motion prediction of a prediction unit of acurrent picture, and when bidirectional motion prediction is performed,2 or 3 is allocated as a value of a reference syntax (Ref Syntax), andone of 0 through 3 is encoded, as motion information, according to aprediction mode and a reference picture applied to a current predictionunit.

At a decoder's end, when the first reference picture list List 0includes two sheets of reference pictures, the second reference picturelist List 1 includes only one sheet of reference picture (L0=2, L1=1),and one of L0 picture and L1 picture is overlapped, and 0 is received asa reference syntax (Ref Syntax), unidirectional motion prediction isperformed using a first reference picture of the two reference picturesincluded in the combination reference picture list, and when 1 isreceived as a reference syntax (Ref Syntax), unidirectional motionprediction is performed using a second reference picture of the tworeference pictures included in the combination reference picture list.Also, at the decoder's end, when 2 is received as a reference syntax(Ref Syntax), a reference picture of a first reference picture indexL0_idx in the first reference picture list List 0 is determined as L0picture, and a reference picture in the second reference picture listList 1 is determined as L1 picture, and then bidirectional motionprediction is performed. At the decoder's end, when 3 is received as areference syntax (Ref Syntax), a reference picture of a second referencepicture index L1_idx is determined as L0 picture in the first referencepicture list List 0, and the reference picture in the second referencepicture list List 1 is determined as L1 picture, and then bidirectionalmotion prediction is performed.

d) When MaxVal=3 (Reference Numeral 1740)

When the first reference picture list List 0 includes only one sheet ofa reference picture, and the second reference picture list List 1includes two sheets of reference pictures (L0=1, L1=2), and one of L0picture and L1 picture is overlapped, a total of two sheets of referencepictures are included in a combination reference picture list.Accordingly, two cases of unidirectional motion prediction where twosheets of reference pictures are used exist. In addition, two cases((0,0), (1,0)) of bidirectional motion prediction that is performed byreferring to each reference picture (L0 and L1 pictures) of the firstreference picture list List 0 and the second reference picture list List1 exist.

Accordingly, the entropy encoder 1410 allocates 0 or 1 as a value of areference syntax (Ref Syntax) according to which reference picture isused during unidirectional motion prediction of a prediction unit of acurrent picture, and when bidirectional motion prediction is performed,2 or 3 is allocated as a value of a reference syntax (Ref Syntax), andone of 0 through 3 is encoded, as motion information, according to aprediction mode and a reference picture applied to the currentprediction unit.

At a decoder's end, when the first reference picture list List 0includes one sheet of a reference picture, and the second referencepicture list List 1 includes two sheets of reference pictures (L0=1,L1=2), one of L0 picture and L1 picture is overlapped, and 0 is receivedas a reference syntax (Ref Syntax), unidirectional motion prediction isperformed using a first reference picture of the two reference picturesincluded in the combination reference picture list, and when 1 isreceived as a reference syntax (Ref Syntax), unidirectional motionprediction is performed using a second reference picture of the tworeference pictures included in the combination reference picture list.Also, at the decoder's end, when 2 is received as a reference syntax(Ref Syntax), one sheet of a reference picture in the first referencepicture list List 0 is determined as L0 picture, and a first referencepicture in the second reference picture list List 1 is determined as L1picture, and then bidirectional motion prediction is performed. At thedecoder's end, when 3 is received as a reference syntax (Ref Syntax),one sheet of reference picture in the first reference picture list List0 is determined as L0 picture, and a second reference picture in thesecond reference picture list List 1 is determined as L1 picture, andthen bidirectional motion prediction is performed.

e) When MaxVal=5 (Reference Numeral 1750)

When the first reference picture list List 0 includes two referencepictures, and the second reference picture list List 1 includes only onesheet of a reference picture (L0=2, L1=1), and there is no overlappingpicture among L0 picture and L1 picture, a total of three sheets ofreference pictures are included in a combination reference picture list.Accordingly, three cases of unidirectional motion prediction where threesheets of reference pictures are used exist. In addition, two cases,((0,0), (1,0)), of bidirectional motion prediction that is performed byreferring to each reference picture (L0 and L1 pictures) of the firstreference picture list List 0 and the second reference picture list List1 exist.

Accordingly, the entropy encoder 1410 allocates one of 0 through 2 as avalue of a reference syntax (Ref Syntax) according to which referencepicture is used during unidirectional motion prediction of a predictionunit of a current picture, and when bidirectional motion prediction isperformed, 3 or 4 is allocated as a value of a reference syntax (RefSyntax), and one of 0 through 4 is encoded, as motion information,according to a prediction mode and a reference picture applied to acurrent prediction unit.

At a decoder's end, when the first reference picture list List 0includes two sheets of reference pictures, and the second referencepicture list List 1 includes only one sheet of reference picture (L0=2,L1=1), there is no overlapping picture among L0 picture and L1 picture,and 0 is received as a reference syntax (Ref Syntax), unidirectionalmotion prediction is performed using a first reference picture of thethree reference pictures included in the combination reference picturelist; when 1 is received as a reference syntax (Ref Syntax),unidirectional motion prediction is performed using a second referencepicture of the three reference pictures included in the combinationreference picture list; and when 2 is received as a reference syntax(Ref Syntax), unidirectional motion prediction is performed using athird reference picture of the three reference pictures included in thecombination reference picture list.

Also, at the decoder's end, when 3 is received as a reference syntax(Ref Syntax), a reference picture of a first reference picture indexL0_idx in the first reference picture list List 0 is determined as L0picture, and one sheet of a reference picture in the second referencepicture list List 1 is determined as L1 picture, and then bidirectionalmotion prediction is performed. At the decoder's end, when 4 is receivedas a reference syntax (Ref Syntax), a reference picture of a secondreference picture index L1_idx in the first reference picture list List0 is determined as L0 picture, and one sheet of reference picture in thesecond reference picture list List 1 is determined as L1 picture, andthen bidirectional motion prediction is performed.

f) When MaxVal=4 (Reference Numeral 1760)

When the first reference picture list List 0 includes only one sheet ofa reference picture, and the second reference picture list List 1includes two sheets of reference pictures (L0=1, L1=2), and there is nooverlapping picture among L0 picture and L1 picture, a total of threesheets of reference pictures are included in a combination referencepicture list. Accordingly, three cases of unidirectional motionprediction where three sheets of reference pictures are used exist. Inaddition, two cases ((0,0), (1,0)) of bidirectional motion predictionthat is performed by referring to each reference picture (L0 and L1pictures) of the first reference picture list List 0 and the secondreference picture list List 1 exist.

Accordingly, the entropy encoder 1410 allocates one of 0 through 2 as avalue of a reference syntax (Ref Syntax) according to which referencepicture is used during unidirectional motion prediction of a predictionunit of a current picture, and when bidirectional motion prediction isperformed, 3 or 4 is allocated as a value of a reference syntax (RefSyntax), and one of 0 through 4 is encoded, as motion information,according to a prediction mode and a reference picture applied to acurrent prediction unit.

At a decoder's end, when the first reference picture list List 0includes two sheets of reference pictures, and the second referencepicture list List 1 includes only one sheet of reference picture (L0=2,L1=1), there is no overlapping picture among L0 picture and L1 picture,and 0 is received as a reference syntax (Ref Syntax), unidirectionalmotion prediction is performed using a first reference picture of thethree reference pictures included in the combination reference picturelist; when 1 is received as a reference syntax (Ref Syntax),unidirectional motion prediction is performed using a second referencepicture of the three reference pictures included in the combinationreference picture list; and when 2 is received as a reference syntax(Ref Syntax), unidirectional motion prediction is performed using athird reference picture of the three reference pictures included in thecombination reference picture list.

Also, at the decoder's end, when 3 is received as a reference syntax(Ref Syntax), one sheet of reference picture of in the first referencepicture list List 0 is determined as L0 picture, and a first referencepicture in the second reference picture list List 1 is determined as L1picture, and then bidirectional motion prediction is performed. At thedecoder's end, when 4 is received as a reference syntax (Ref Syntax),one sheet of a reference picture in the first reference picture listList 0 is determined as L0 picture, and a second reference picture inthe second reference picture list List 1 is determined as L1 picture,and then bidirectional motion prediction is performed.

g) When MaxVal=5 (Reference Numeral 1770)

When the first reference picture list List 0 and the second referencepicture list List 1 each include two sheets of reference pictures (L0=2,L1=2), and two reference pictures of L0 picture and L1 picture areoverlapped, a total of two sheets of reference pictures are included ina combination reference picture list. Thus, two cases of unidirectionalmotion prediction that is performed using two sheets of referencepictures exist. In addition, four cases ((0,0), (0,1), (1,0), (1,1)) ofbidirectional motion prediction that is performed using each referencepicture (L0 and L0 pictures) of the first reference picture list List 0and the second reference picture list List 1 exist.

Accordingly, the entropy encoder 1410 allocates 0 or 1 as a value of areference syntax (Ref Syntax) according to which reference picture isused during unidirectional motion prediction of a prediction unit of acurrent picture, and when bidirectional motion prediction is performed,one of 2 through 5 is allocated as a value of a reference syntax (RefSyntax), and one of 0 through 5 is encoded, as motion information,according to a prediction mode and a reference picture applied to acurrent prediction unit.

At a decoder's end, when the first reference picture list List 0 and thesecond reference picture list List 1 each includes two sheets ofreference pictures (L0=2, L1=2), two sheets of reference pictures areoverlapped among L0 picture and L1 picture, and 0 is received as areference syntax (Ref Syntax), unidirectional motion prediction isperformed using a first reference picture of the two reference picturesincluded in the combination reference picture list, and when 1 isreceived as a reference syntax (Ref Syntax), unidirectional motionprediction is performed using a second reference picture of the tworeference pictures included in the combination reference picture list.

Also, at the decoder's end, when 2 is received as a reference syntax(Ref Syntax), a first reference picture in the first reference picturelist List 0 is determined as L0 picture, and a first reference picturein the second reference picture list List 1 is determined as L1 picture,and then bidirectional motion prediction is performed. At the decoder'send, when 3 is received as a reference syntax (Ref Syntax), the firstreference picture in the first reference picture list List 0 isdetermined as L0 picture, and a second reference picture in the secondreference picture list List 1 is determined as L1 picture, and thenbidirectional motion prediction is performed. At the decoder's end, when4 is received as a reference syntax (Ref Syntax), a second referencepicture in the first reference picture list List 0 is determined as L0picture, and the first reference picture in the second reference picturelist List 1 is determined as L1 picture, and then bidirectional motionprediction is performed. At the decoder's end, when 5 is received as areference syntax (Ref Syntax), the second reference picture in the firstreference picture list List 0 is determined as L0 picture, and thesecond reference picture in the second reference picture list List 1 isdetermined as L1 picture, and then bidirectional motion prediction isperformed.

h) When MaxVal=6 (Reference Numeral 1780)

When the first reference picture list List 0 and the second referencepicture list List 1 each include two sheets of reference pictures (L0=2,L1=2), and one reference picture among L0 picture and L1 picture isoverlapped, a total of three sheets of reference pictures are includedin a combination reference picture list. Thus, three cases ofunidirectional motion prediction which is performed by using threesheets of reference pictures exist. In addition, four cases ((0,0),(0,1), (1,0), (1,1)) of bidirectional motion prediction that isperformed using each reference picture L0 and L0 pictures of the firstreference picture list List 0 and the second reference picture list List1 exist.

Accordingly, the entropy encoder 1410 allocates one of 0 through 2 as avalue of a reference syntax (Ref Syntax) according to which referencepicture is used during unidirectional motion prediction of a predictionunit of a current picture, and when bidirectional motion prediction isperformed, one of 3 through 6 is allocated as a value of a referencesyntax (Ref Syntax), and one of 0 through 6 is encoded, as motioninformation, according to a prediction mode and a reference pictureapplied to a current prediction unit.

At a decoder's end, when the first reference picture list List 0 and thesecond reference picture list List 1 each include two sheets ofreference pictures (L0=2, L1=2), one sheet of L0 picture and L1 pictureis overlapped, when 0 is received as a reference syntax (Ref Syntax),unidirectional motion prediction is performed using a first referencepicture of the three reference pictures included in the combinationreference picture list; when 1 is received as a reference syntax (RefSyntax), unidirectional motion prediction is performed using a secondreference picture of the three reference pictures included in thecombination reference picture list; and when 2 is received as areference syntax (Ref Syntax), unidirectional motion prediction isperformed using a third reference picture of the three referencepictures included in the combination reference picture list.

At the decoder's end, when 3 is received as a reference syntax (RefSyntax), a first reference picture in the first reference picture listList 0 is determined as L0 picture, and a first reference picture in thesecond reference picture list List 1 is determined as L1 picture, andthen bidirectional motion prediction is performed. At the decoder's end,when 4 is received as a reference syntax (Ref Syntax), the firstreference picture in the first reference picture list List 0 isdetermined as L0 picture, and a second reference picture in the secondreference picture list List 1 is determined as L1 picture, and thenbidirectional motion prediction is performed. At the decoder's end, when5 is received as a reference syntax (Ref Syntax), a second referencepicture in the first reference picture list List 0 is determined as L0picture, and the first reference picture in the second reference picturelist List 1 is determined as L1 picture, and then bidirectional motionprediction is performed. At the decoder's end, when 6 is received as areference syntax (Ref Syntax), the second reference picture in the firstreference picture list List 0 is determined as L0 picture, and thesecond reference picture in the second reference picture list List 1 isdetermined as L1 picture, and then bidirectional motion prediction isperformed.

i) When MaxVal=7 (Reference Numeral 1790)

When the first reference picture list List 0 and the second referencepicture list List 1 each include two sheets of reference pictures (L0=2,L1=2), and there is no overlapping picture in the L0 picture and the L1picture, a total of four sheets of reference pictures are included in acombination reference picture list. Thus, four cases of unidirectionalmotion prediction which is performed by using four sheets of referencepictures exist. In addition, four cases ((0,0), (0,1), (1,0), (1,1)) ofbidirectional motion prediction that is performed using each referencepicture (L0 and L0 pictures) of the first reference picture list List 0and the second reference picture list List 1 exist.

Accordingly, the entropy encoder 1410 allocates one of 0 through 3 as avalue of a reference syntax (Ref Syntax) according to which referencepicture is used during unidirectional motion prediction of a predictionunit of a current picture, and when bidirectional motion prediction isperformed, one of 4 through 7 is allocated as a value of a referencesyntax (Ref Syntax), and one of 0 through 7 is encoded, as motioninformation, according to a prediction mode and a reference pictureapplied to a current prediction unit.

At a decoder's end, when the first reference picture list List 0 and thesecond reference picture list List 1 each include two sheets ofreference pictures (L0=2, L1=2), there is no overlapping picture in L0picture and L1 picture, and 0 is received as a reference syntax (RefSyntax), unidirectional motion prediction is performed using a firstreference picture of the four reference pictures included in thecombination reference picture list; when 1 is received as a referencesyntax (Ref Syntax), unidirectional motion prediction is performed usinga second reference picture of the four reference pictures included inthe combination reference picture list; when 2 is received as areference syntax (Ref Syntax), unidirectional motion prediction isperformed using a third reference picture of the four reference picturesincluded in the combination reference picture list; and when 3 isreceived as a reference syntax (Ref Syntax), unidirectional motionprediction is performed using a fourth reference picture of the fourreference pictures included in the combination reference picture list.

At the decoder's end, when 4 is received as a reference syntax (RefSyntax), a first reference picture in the first reference picture listList 0 is determined as L0 picture, and a first reference picture in thesecond reference picture list List 1 is determined as L1 picture, andthen bidirectional motion prediction is performed. At the decoder's end,when 5 is received as a reference syntax (Ref Syntax), the firstreference picture in the first reference picture list List 0 isdetermined as L0 picture, and a second reference picture in the secondreference picture list List 1 is determined as L1 picture, and thenbidirectional motion prediction is performed. At the decoder's end, when6 is received as a reference syntax (Ref Syntax), a second referencepicture in the first reference picture list List 0 is determined as L0picture, and the first reference picture in the second reference picturelist List 1 is determined as L1 picture, and then bidirectional motionprediction is performed. At the decoder's end, when 7 is received as areference syntax (Ref Syntax), the second reference picture in the firstreference picture list List 0 is determined as L0 picture, and thesecond reference picture in the second reference picture list List 1 isdetermined as L1 picture, and then bidirectional motion prediction isperformed.

As described above, the entropy encoder 450 allocates one of 0 through(MaxValue−1) for each combination of reference pictures available in aunidirectional motion prediction mode and reference pictures availablein a bidirectional motion prediction mode, and encodes a value of areference syntax corresponding to a relevant motion prediction mode andreference pictures that are applied to the current prediction unit,thereby encoding a motion prediction mode of a current prediction unitand reference picture information, by using a single reference syntax.

That is, the entropy encoder 450 may allocate values from 0 to(NumOfRef_LC−1) according to a reference picture index in a combinationreference picture list used as prediction mode information and referencepicture information of a current prediction unit that isunidirectionally motion predicted, based on a number of referencepictures NumOfRef_LC in the combination reference picture list, therebyencoding motion information of a current prediction unit. Also, theentropy encoder 450 may allocate values from NumOfRef_LC through(MaxValue−1) and encode motion information of a current prediction unitaccording to which of first and second reference pictures in the firstreference picture list List 0 and the second reference picture list List1 are used, as prediction mode information and reference pictureinformation of the current prediction unit that is bidirectionallymotion predicted.

Also, when a reference syntax (Ref Syntax) has a value MaxValue, theentropy encoder 450 may set an exceptional case which is not a casewhere a motion prediction mode and a reference picture are indicated byusing a preset reference syntax.

The entropy encoder 450 may binarize a reference syntax (Ref Syntax) byusing a truncated unary binarization method to generate a bitstream.

FIG. 18 illustrates an example of a process of binarizing referencesyntax information, according to an exemplary embodiment.

Referring to FIG. 18, the entropy encoder 450 may binarize a value of areference syntax by using a truncated unary binarization method whenencoding a reference syntax (Ref Syntax). That is, the entropy encoder450 may output a number of “1” bits corresponding to a reference syntaxvalue (Value) as illustrated in FIG. 17 and a terminating “0” bit,thereby binarizing the reference syntax. When a reference syntax is notdefined in advance by a reference syntax allocation table as illustratedin FIG. 17, the entropy encoder 450 may indicate this exceptional casewhich is not defined in advance, by outputting a binarization bit stringconsisting of “1” bits of a number corresponding to (MaxValue−1). Forexample, when a total number of cases (MaxValue) of unidirectionalmotion prediction modes and bidirectional motion prediction modes thatare preset by using a reference syntax is 7, and a current predictionunit is set by the syntax reference, the entropy encoder 450 outputs abinarization bit string consisting of consecutive “1” bits and a single“0” bit following the “1” bits as illustrated in FIG. 18 according to avalue of a reference syntax corresponding to a current prediction unit.The entropy encoder 450 may set a reference syntax (Ref Syntax) having avalue of 7 as an exceptional case where a preset combination of aunidirectional prediction mode and a bidirectional motion predictionmode is not used, and outputs “1111111” indicating this exceptionallyprocessed information.

FIG. 19 is a flowchart illustrating an image encoding method accordingto an exemplary embodiment.

Referring to FIG. 19, in operation 1910, the entropy encoder 450 obtainsa first reference picture list, a second reference picture list, and acombination reference picture list which is a combination of referencepictures included in the first reference picture list and referencepictures included in the second reference picture list. As describedabove, when a number of reference pictures included in the firstreference picture list is NumOfRef_L0, a number of reference picturesincluded in the first reference picture list is NumOfRef_L1, and anumber of reference pictures that are overlapped between the first andsecond reference picture lists is NumOfRedundancy, the combinationreference picture list may include non-overlapping reference pictures ofa number of NumOfRef_L0+NumOfRef_L1−NumOfRedundancy.

In operation 1920, the motion estimator 420 performs unidirectionalmotion prediction with respect to a current prediction unit by using areference picture included in the combination reference picture list andbidirectional motion prediction with respect to the current predictionunit by using the first reference picture list and the second referencepicture list, and determines a motion prediction mode having a lowercost, as a prediction mode of the current prediction unit.

In operation 1930, the entropy encoder 450 encodes a reference syntaxindicating a motion prediction mode and a reference picture used inencoding the current prediction unit based on a number of cases of aunidirectional motion prediction mode and a number of cases of abidirectional motion prediction mode. The entropy encoder 450 mayallocate values from 0 to (NumOfRef_LC−1) according to a referencepicture index in a combination reference picture list used as predictionmotion information and reference picture information of the currentprediction unit that is unidirectionally motion predicted, therebyencoding motion information of the current prediction unit. Also, theentropy encoder 450 may allocate values from NumOfRef_LC to (MaxValue−1)according to which of a first reference picture and a second referencepicture in the first reference picture list List 0 and the secondreference picture list List 1 is used as prediction mode information andreference picture information of the current prediction unit that isbidirectionally motion predicted, thereby encoding motion information ofthe current prediction unit. Also, when a reference syntax (Ref Syntax)has a value MaxValue, the entropy encoder 450 may encode motioninformation of the current prediction unit for setting an exceptionalcase which does not correspond to neither a preset unidirectionalprediction mode nor a preset bidirectional motion prediction mode.

During decoding, the entropy decoder 520 of FIG. 5 obtains a firstreference picture list, a second reference picture list, and acombination reference picture list which is a combination of referencepictures included in the first reference picture list and referencepictures included in the second reference picture list, and determines avalue of a reference syntax according to a motion prediction mode andreference pictures used in encoding a current prediction unit based on anumber of cases of a unidirectional motion prediction mode wherereference pictures included in the combination reference list are usedand a number of cases of a bidirectional motion prediction mode wherethe first reference picture list and the second reference picture listare used.

In detail, in the same manner as the entropy encoder 450 describedabove, the entropy decoder 520 calculates a sum MaxValue of the numberof cases of the unidirectional motion prediction mode and the number ofcases of the bidirectional motion prediction mode according to thefollowing equation: MaxValue=NumOfRef_LC+NumOfRef_L0*NumOfRef_L1, andthen when a reference syntax (Ref Syntax) is a value from 0 to(NumOfRef_LC−1), unidirectional motion prediction is indicated byreferring to a reference picture from among combination referencepicture lists of a number NumOfRef_LC, and when the reference syntax hasa value from NumOfRef_LC to (MaxValue−1), a bidirectional motionprediction mode, where two reference pictures according to a combinationof a first reference picture of the first reference picture list and asecond reference picture of the second reference picture list are used,is determined as a motion prediction mode. As described above,information about which prediction mode among a unidirectionalprediction mode and a bidirectional motion prediction mode is applied toprediction of a current prediction unit and information about referencepictures used in the prediction of the current unit may be determinedbased on the reference syntax itself.

The motion compensator 560 may use prediction mode information andreference picture determined based on the reference syntax of thecurrent prediction unit obtained by using the entropy decoder 520 toperform unidirectional motion compensation and bidirectional motioncompensation with respect to the current prediction unit, therebygenerating a prediction value of the current prediction unit.

FIG. 20 is a flowchart illustrating an image decoding method accordingto an exemplary embodiment.

Referring to FIG. 20, in operation 2010, the entropy decoder 520 obtainsa first reference picture list, a second reference picture list, and acombination reference picture list which is a combination of referencepictures included in the first reference picture list and referencepictures included in the second reference picture list. In operation2020, the entropy decoder 520 determines a value of a reference syntaxaccording to a motion prediction mode and reference pictures used inencoding the current prediction unit based on a number of cases of aunidirectional motion prediction mode where reference pictures includedin the combination reference list are used and a number of cases of abidirectional motion prediction mode where the first reference picturelist and the second reference picture list are used.

In operation 2030, the entropy decoder 520 obtains a reference syntax ofthe current prediction unit from a bitstream. As described above, thereference syntax may be encoded by a truncated unary binarizationmethod, and it may be determined which case among the number of cases ofprediction modes illustrated in FIG. 17 is indicated by the currentreference syntax based on the whole number of cases (MaxValue) ofprediction modes.

In operation 2040, the entropy decoder 520 may determine a motionprediction mode and a reference picture of the current prediction unitby using a value of the reference syntax, and in operation 2050, themotion compensator 560 may perform motion compensation with respect tothe current prediction unit by using the determined motion predictionmode and the determined reference picture to generate a prediction valueof the current prediction unit.

Exemplary embodiments may also be embodied as computer readable codes ona computer readable recording medium. The computer readable recordingmedium is any data storage device that may store data which bethereafter read by a computer system. Examples of the computer readablerecording medium include read-only memory (ROM), random-access memory(RAM), compact disc read-only memories (CD-ROMs), magnetic tapes, floppydisks, optical data storage devices, etc. The computer readablerecording medium may also be distributed over network coupled computersystems so that the computer readable code is stored and executed in adistributed fashion.

It is understood that, although particular embodiments have just beendescribed, the disclosure is not limited in scope to particularembodiments or implementation. For example, one embodiment may beimplemented in hardware, whereas another embodiment may be implementedin software Likewise, an embodiment may be implemented in firmware, orany combination of hardware, software, or firmware, for example.

According to exemplary embodiments, prediction direction (predictionmode) information and reference picture information used for a currentprediction unit may be efficiently encoded by using a single referencesyntax, thereby improving compression efficiency of an image.

While the disclosure has been particularly shown and described withreference to exemplary embodiments thereof, it will be understood bythose of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the disclosure as defined by the appended claims. Therefore, thescope of the disclosure is defined not by the detailed description ofthe disclosure but by the appended claims, and all differences withinthe scope will be construed as being included in the disclosure.

1. An image encoding method comprising: obtaining a first referencepicture list, a second reference picture list, and a combinationreference picture list which is a combination of reference picturesincluded in the first reference picture list and reference picturesincluded in the second reference picture list; encoding a currentprediction unit by using one of a unidirectional prediction mode inwhich unidirectional prediction is performed with respect to the currentprediction unit by using a reference picture included in the combinationreference picture list and a bidirectional prediction mode in whichbidirectional motion prediction is performed with respect to the currentprediction unit by using a combination of reference pictures included inthe first reference picture list and the second reference picture list;and encoding a reference syntax indicating a motion prediction mode anda reference picture used in encoding the current prediction unit basedon a number of possible cases of the unidirectional motion predictionmode and a number of possible cases of the bidirectional motionprediction mode.
 2. The image encoding method of claim 1, wherein, inthe encoding the reference syntax, a reference syntax of a differentvalue is allocated to each of reference pictures included in thecombination reference picture list available in the unidirectionalmotion prediction mode and each combination of reference picturesincluded in the first reference picture list and the second referencepicture list available in the bidirectional motion prediction mode, anda value of a reference syntax corresponding to a motion prediction modeof the current prediction unit and reference pictures used in motionprediction of the current prediction unit is encoded.
 3. The imageencoding method of claim 2, wherein, when a number of reference picturesincluded in the combination reference list is NumOfRef_LC, a number ofreference pictures included in the first reference picture list isNumOfRef_L0, a number of reference pictures included in the secondreference picture list is NumOfRef_L1, and a total number of possiblecases of unidirectional and bidirectional motion prediction of thecurrent prediction unit is MaxValue, MaxValue has a value determinedaccording to the following equation:Max-Value=NumOfRef_LC+NumOfRef_L0*NumOfRef_L1, wherein one of valuesfrom 0 through (Max-Value−1) is allocated as the reference syntax forthe each of the reference pictures available in the unidirectionalmotion prediction mode and the each combination of the referencepictures available in the bidirectional motion prediction mode.
 4. Theimage encoding method of claim 3, wherein, when a number of referencepictures that are overlapped in the first reference picture list and thesecond reference picture list is NumOfRedundancy, non-overlappingreference pictures corresponding to a number ofNumOfRef_L0+NumOfRef_L1−NumOfRedundancy are included in the combinationreference picture list.
 5. The image encoding method of claim 3,wherein, when the current prediction unit is unidirectionally motionpredicted, a value in a range from 0 to (NumOfRef_LC−1) is encoded asthe reference syntax of the current prediction unit according to areference picture of the combination reference picture list that isreferred to by the current prediction unit.
 6. The image encoding methodof claim 3, wherein, when the current prediction unit is bidirectionallymotion predicted, a value in a range from NumOfRef_LC to (MaxValue−1) isencoded as the reference syntax of the current prediction unit accordingto a combination of a reference picture of the first reference picturelist and a reference picture of the second reference picture list thatare referred to by the current prediction unit.
 7. The image encodingmethod of claim 3, wherein, when the reference syntax has a value ofMaxValue, an exceptional case where a reference picture of theunidirectional motion prediction mode and a combination of referencepictures of the bidirectional motion prediction mode is not used isindicated.
 8. An image decoding method comprising: obtaining a firstreference picture list, a second reference picture list, and acombination reference picture list which is a combination of referencepictures included in the first reference picture list and referencepictures included in the second reference picture list; determining avalue of a reference syntax according to a motion prediction mode andreference pictures used in encoding a current prediction unit based on anumber of possible cases of a unidirectional motion prediction modewhere reference pictures included in the combination reference list areused and a number of possible cases of a bidirectional motion predictionmode where a combination of reference pictures included in the firstreference picture list and the second reference picture list is used;obtaining a reference syntax of the current prediction unit from abitstream; determining a motion prediction mode and a reference pictureof the current prediction unit by using a value of the obtainedreference syntax; and performing motion compensation with respect to thecurrent prediction unit by using the determined motion prediction modeand the determined reference picture.
 9. The image decoding method ofclaim 8, wherein, in the first reference picture list, a referencepicture index is allocated in an order from a past reference picturethat is closest to a current picture to a reference picture previous tothe past reference picture, and in the second reference picture list, areference picture index is allocated in an order from a future referencepicture that is closest to the current picture to a reference picturesubsequent to the future reference picture.
 10. The image decodingmethod of claim 8, wherein the reference syntax has a value allocated toeach of reference pictures included in the combination reference picturelist that are available in the unidirectional motion prediction mode andeach combination of reference pictures included in the first referencepicture list and the second reference picture list that are available inthe bidirectional motion prediction mode.
 11. The image decoding methodof claim 10, wherein, when a number of reference pictures included inthe combination reference list is NumOfRef_LC, a number of referencepictures included in the first reference picture list is NumOfRef_L0, anumber of reference pictures included in the first reference picturelist is NumOfRef_L1, and a total number of possible cases ofunidirectional and bidirectional motion prediction of the currentprediction unit is MaxValue, MaxValue has a value determined accordingto the following equation:Max-Value=NumOfRef_LC+NumOfRef_L0*NumOfRef_L1, wherein one of valuesfrom 0 through (Max-Value−1) is allocated as the reference syntax forthe each of the reference pictures available in the unidirectionalmotion prediction mode and the each combination of the referencepictures available in the bidirectional motion prediction mode.
 12. Theimage decoding method of claim 11, wherein, when a number of referencepictures that are overlapped in the first reference picture list and thesecond reference picture list is NumOfRedundancy, non-overlappingreference pictures corresponding to a number ofNumOfRef_L0+NumOfRef_L1−NumOfRedundancy are included in the combinationreference picture list.
 13. The image decoding method of claim 11,wherein it is determined that the current prediction unit isunidirectionally motion predicted upon receiving a value in a range from0 to (NumOfRef_LC−1) of the reference syntax of the current predictionunit.
 14. The image decoding method of claim 11, wherein it isdetermined that the current prediction unit is bidirectionally motionpredicted upon receiving a value in a range from NumOfRef_LC to(MaxValue−1) of the reference syntax of the current prediction unit. 15.The image decoding method of claim 11, wherein, when the referencesyntax has a value of MaxValue, an exceptional case where a referencepicture of the unidirectional motion prediction mode and a combinationof reference pictures of the bidirectional motion prediction mode is notused is indicated.
 16. An image encoding apparatus, comprising: a motionestimator configured to predict a current prediction unit by using oneof a unidirectional prediction mode where unidirectional motionprediction with respect to a current prediction unit is performed byusing a reference picture included in a combination reference picturelist in which reference pictures of a first reference picture list andreference pictures of a second reference picture list are combined, anda bidirectional motion prediction mode where bidirectional motionprediction with respect to the current prediction unit is performed byusing a combination of reference pictures included in the firstreference picture list and the second reference picture list; and anentropy encoder configured to encode a reference syntax indicating amotion prediction mode and a reference picture used in encoding thecurrent prediction unit based on a number of possible cases of theunidirectional motion prediction mode and a number of possible cases ofthe bidirectional motion prediction mode.
 17. The image encodingapparatus of claim 16, wherein the entropy encoder allocates a referencesyntax of a different value to each of reference pictures included inthe combination reference picture list available in the unidirectionalmotion prediction mode and each combination of reference picturesincluded in the first reference picture list and the second referencepicture list available in the bidirectional motion prediction mode, andencodes a value of a reference syntax corresponding to a motionprediction mode of the current prediction unit and reference picturesused in motion prediction of the current prediction unit.
 18. The imageencoding apparatus of claim 17, wherein, when a number of referencepictures included in the combination reference list is NumOfRef_LC, anumber of reference pictures included in the first reference picturelist is NumOfRef_L0, a number of reference pictures included in thesecond reference picture list is NumOfRef_L1, and a total number ofpossible cases of unidirectional and bidirectional motion prediction ofthe current prediction unit is MaxValue, an entropy encoder determines avalue MaxValue according to the following equation:MaxValue=NumOfRef_LC+NumOfRef_L0*NumOfRef_L1, wherein the entropyencoder allocates one of values from 0 through (MaxValue−1) as thereference syntax for the each of the reference pictures available in theunidirectional motion prediction mode and the each combination of thereference pictures available in the bidirectional motion predictionmode.
 19. The image encoding apparatus of claim 18, wherein, when anumber of reference pictures that are overlapped in the first referencepicture list and the second reference picture list is NumOfRedundancy,non-overlapping reference pictures corresponding to a number ofNumOfRef_L0+NumOfRef_L1−NumOfRedundancy are included in the combinationreference picture list.
 20. The image encoding apparatus of claim 18,wherein, when the current prediction unit is unidirectionally motionpredicted, the entropy encoder encodes a value in a range from 0 to(NumOfRef_LC−1) as the reference syntax of the current prediction unitaccording to a reference picture of the combination reference picturelist that is referred to by the current prediction unit.
 21. The imageencoding apparatus of claim 18, wherein, when the current predictionunit is bidirectionally motion predicted, the entropy encoder encodes avalue in a range from NumOfRef_LC to (MaxValue−1) as the referencesyntax of the current prediction unit according to a combination of areference picture of the first reference picture list and a referencepicture of the second reference picture list that are referred to by thecurrent prediction unit.
 22. The image encoding apparatus of claim 18,wherein, when the reference syntax has a value of MaxValue, anexceptional case where a reference picture of the unidirectional motionprediction mode and a combination of reference pictures of thebidirectional motion prediction mode is not used is indicated.
 23. Animage decoding apparatus, comprising: an entropy decoder configured toobtain a first reference picture list, a second reference picture list,and a combination reference picture list which is a combination ofreference pictures included in the first reference picture list andreference pictures included in the second reference picture list,determine a value of a reference syntax according to a motion predictionmode and reference pictures used in encoding a current prediction unitbased on a number of possible cases of a unidirectional motionprediction mode where reference pictures included in the combinationreference list are used and a number of possible cases of abidirectional motion prediction mode where a combination of referencepictures included in the first reference picture list and the secondreference picture list is used, and determine a motion prediction modeand a reference picture of the current prediction unit by using areference syntax of the current prediction unit obtained from abitstream; and a motion compensation unit configured to perform motioncompensation with respect to the current prediction unit by using thedetermined motion prediction mode and the determined reference picture.24. The image decoding apparatus of claim 23, wherein, in the firstreference picture list, a reference picture index is allocated in anorder from a past reference picture that is closest to a current pictureto a reference picture previous to the past reference picture, and inthe second reference picture list, a reference picture index isallocated in an order from a future reference picture that is closest tothe current picture to a reference picture subsequent to the futurereference picture.
 25. The image decoding apparatus of claim 23, whereinthe reference syntax has a value allocated to each of reference picturesincluded in the combination reference picture list that are available inthe unidirectional motion prediction mode and each combination ofreference pictures included in the first reference picture list and thesecond reference picture list that are available in the bidirectionalmotion prediction mode.
 26. The image decoding apparatus of claim 25,wherein, when a number of reference pictures included in the combinationreference list is NumOfRef_LC, a number of reference pictures includedin the first reference picture list is NumOfRef_L0, a number ofreference pictures included in the first reference picture list isNumOfRef_L1, and a total number of possible cases of unidirectional andbidirectional motion prediction of the current prediction unit isMaxValue, the entropy decoder determines a value MaxValue according tothe following equation: MaxValue=NumOfRef_LC+NumOfRef_L0*NumOfRef_L1,wherein one of values from 0 through (MaxValue−1) is allocated as thereference syntax for the each of the reference pictures available in theunidirectional motion prediction mode and the each combination of thereference pictures available in the bidirectional motion predictionmode.
 27. The image decoding apparatus of claim 26, wherein, when anumber of reference pictures that are overlapped in the first referencepicture list and the second reference picture list is NumOfRedundancy,non-overlapping reference pictures corresponding to a number ofNumOfRef_L0+NumOfRef_L1−NumOfRedundancy are included in the combinationreference picture list.
 28. The image decoding apparatus of claim 26,wherein the entropy decoder determines that the current prediction unitis unidirectionally motion predicted upon receiving a value in a rangefrom 0 to (NumOfRef_LC−1) of the reference syntax of the currentprediction unit.
 29. The image decoding apparatus of claim 26, whereinthe entropy decoder determines that the current prediction unit isbidirectionally motion predicted upon receiving a value in a range fromNumOfRef_LC to (MaxValue−1) of the reference syntax of the currentprediction unit.
 30. The image decoding apparatus of claim 26, wherein,when the reference syntax has a value of MaxValue, an exceptional casewhere a reference picture of the unidirectional motion prediction modeand a combination of reference pictures of the bidirectional motionprediction mode is not used is indicated.